Not necessarily.
In FM4 I drove a bunch of fast race cars in free play mode before starting career. The same goes for PCars, I started with the Formula A in solo mode, then drove the other formula cars, then some LMPs, then GTs, I could drive them all just fine. Having all cars available from the start in solo mode made the game more enjoyable.
The way I see it one of the reasons we see so many bad players online in GT6 is because of its offline career format.
The catch-up race format encourages reckless driving, player starts in last place far behind the leader, tries to pass AI cars at any cost with no damage and no penalties for hitting other cars or going offtrack, in an attempt to win every race so that they get the stars to unlock the next career section and credits to acquire cars. There's also the issue of the AI being poor, and race events also being setup poorly, there's usually 1-2 AI drivers using cars that are significantly faster than the other AI cars, but even those 1-2 opponents are using cars well below the performance limit of the event. Most people won't bother doing something like
this and will simply use overpowered cars for an easy win. None of that stuff is realistic or challenging.
When one takes the "skills" learned in GT6's career and uses them online against other human players the result is very poor racing, people try to do the same thing they do in career mode, reckless driving, collisions, corner cutting, bad driving, and if they aren't winning they quit because the game trained them to "win" every race and they don't understand how real life races & championships work.
Having all cars unlocked from the start in arcade mode won't have much effect on how people drive, if PD wants players to race clean they need to do so by implementing ways to encourage clean driving in career (
example) as well as ways to encourage clean driving online (
example).