I'm not sure how you can justify that. GT5 has everything the previous GT has and then some, plus it also does the most important aspects much better than any previous GT. The only thing it's missing is maybe the sheer amount of a spec events, but really that's negated by the seasonals in my opinion.
Plus it's not like GT5 has no a spec events.
I'm quite sure I can justify it, mainly because as its my opinion the only person I have to justify it to is myself. However I will expand.
The main thing that GT5 is missing from the past titles is consistency, we have a game that has a split in the car and track roster in a massive way. So yes 20% of the cars look amazing inside and out, but 80% don't, and its a similar story for the tracks. This has never been the case in any past title, however that's a minor point for me (but not for many).
The A-spec list is down on past titles and no the seasonals do not negate that at all, they are little more than time trials with rolling obstacles. What GT5 misses is a good count of actual races.
To me the combination of the physics being leaps and bounds better than the previous GT titles, the ridiculous amount of replayability it has in comparison, the updated graphics, content, and functionality, and even just the little things like adjustable HUD, cockpit view, and custom soundtracks just push GT5 way over the other GT titles. Not to mention the course maker, which is possibly the most innovative feature put into a console sim in recent memory. Plus, the AI is better, the sound is better, (both of which receive a lot of deserved criticism, but still were an improvement) and another very important aspect racing is physical damage, which we now have.
Now we get onto some of the core here.
The physics are not leaps and bounds better than previous titles, rather some of the old issues from past titles in the series are still present, with a poor tyre model and suspension modeling that seems stuck in GT4. Not only has it not moved things forward dramatically here at all, but its now lagging behind most of the other sims on the market. For that matter its still behind a couple of PS2 sims in regard to certain areas of the physics model.
The course maker is nice, but its certainly not revolutionary (V Rally 2 actually did better back on the PS1 in terms of options and features) and it still doesn't allow us to create point to point events ourselelves, despite quite clearly being able to.
Sound (always a weak point of the series) still is poor and the physical damage is a nice addition, when you can use it.
Another disadvantage GT5 has over previous GT's, is it's definitely less consistent or polished (jagged shadows, 2D Forests on Deep Forest, Trial Mountain, standard cars, crappy visual damage) but I really can't see how that makes it any worse considering the graphics are still a massive improvement and there was no visual damage in the first place. Plus what really matters is the physics, and I don't think anyone would try to argue that the physics aren't better.
The physics are better if you only use the GT series as a bench mark, but they are not cutting it in the market place today and you can't gloss over the issues that GT5 has visually and more importantly how that can affect the frame rate.
The frame rate in GT5 can drop to sub 30fps once you throw in rain and a busy grid, which is simply unacceptable in a racing sim for me.
We also have issues with tuning settings being backwards and the degree of adjustablity not having improved over past releases, hell we couldn't even change gear ratios for the first six months it was out.
We have licence tests that do nothing.
We have an arcade side that feels totally disjointed from the rest of the game.
Expectations should not be a factor here, but they are.
Why shouldn't they be, if they are based on expectations that PD lead people to have.
I think the only reason people are down on GT5 is they expected more from it, and maybe a little nostalgia but that has nothing to do with the quality of the game. The fact is although there were some things to be disappointed about with GT5, but that doesn't change the fact that is a better game. You can call this an opinion but it's more than that because I can quantify why it's better, and what it has that the previous GT's don't have.
You can put together a feature list of all the things that GT5 has that past titles in the series didn't have, and I can put together a list of things that past titles did better than GT5 does.
They are still no more than opinions, you can't tell me that I'm wrong in thinking that GT5 is the weakest title in the series, because it my opinion.
Really I can't see any quantifiable reason that anyone could see GT5 is even not the best GT, let alone the worst. The only reasons I've heard from people that say it's not the best have more to do with their own emotions than the actual game.
An increased feature count doesn't equal a better game, if people don't find them well implemented and poorly designed then they are not a plus but a negative.
I'm pretty sure the reason you think it's the worst has a lot to do with the fact that you've been a fan since GT1.
Why?
Maybe I'm crazy, but I'm just about 100% positive that if you took someone who has never heard of GT, and let them play all 5 games, in any order, that there's no way they wouldn't think GT5's the best. Why? Because they wouldn't have expectations that weren't met, or reasons to be disappointed, or memories of getting more enjoyment out of a previous GT. They would just easily see the huge differences, improvements, and advantages it really does have over previous GT's.
What if I only showed them a standard track with standard cars and then demo'd GT5
to them?
What if they wanted to drive a current generation car in GT5, the list of 2012 models is not exactly strong is it?
What if I had to explain the reason why the interior of a standard car looks the way it does?
I don't expect to convince anyone to change their mind, but I hope they will at least be open to the idea that their emotions maybe clouding their judgement. Not to say emotions don't matter, but they do not effect the quality of a sim, neither do opinions. The sim is what it is, no matter how you perceive it, and this game is the best in the series.
I find it odd that you decry opinions and then finish your post with one.
GT5 may well have the largest car count and feature count in a GT title, but that doesn't mean its the best at all. Personally in terms of overall quality (consistency) handling model (for its age) and feature count I would still go with GT4 and even that had issues. However those issues are not on the scale of the ones GT5 has.
GT5 is for me a jack of all trades, and most certainly not a master of any of them. In trying to throw so many features at GT5, all PD ended up doing is loosing focus on the whole and as a result its a miss-mash of ideas that don't gel and are often not well implemented.