From experience you really can't half-arse physics in a game. It's either sim or arcade. Simcade almost never satisfies either side of the fence. Arcade physics with "sim" mode (or vice versa), almost always never works well either. You really have to commit to one side.
If it was a developer starting entirely from scratch I could see your point but Codemasters aren't, they already have both of the fundamental bases required to have this dual-physics setup. Indeed that's how one of the devs has described the two sides of the game: "Dirt 3 but better, and Dirt Rally but better". They have already proven they can pull off both of these things, and we know they're treating them separately rather than attempting to merge them together. So I don't see why either physics system would suddenly become worse, simply because they happen to exist on the same disc this time.
Track generator is good to an extent, but like GT5's generator after 10 or so tracks everything starts to feel same ol' same ol' with no personality. Sure it gives variety, but it will never have the same cachet as a proper hand designed track.
I can understand that concern but I'd say there's a few key differences. The constraints that govern generating closed-circuit racetracks (like the fact it necessarily must form a closed-loop, or the types of corners and gradient changes that are acceptable, for example) are likely far greater than those for point-to-point rally stages. CM should have more freedom in generation (and hence greater variability) than PD's course maker could ever manage.
Another thing that will naturally help with repetition is, in GT5 the tracks you generate you would lap a number of times. In Dirt 4, a lot of the stages you generate you may only drive once. And if you're spending less time on each generated course, any flaws they may have are probably less likely to stand out.
Then on the personality front, I think the important factor there is the quality of the
scenery generation, rather than the
track generation (unless you think hand-design gives something unique to the actual driving surface that procedural generation couldn't, if so I'd be interested to hear). PD's course maker was not very good at this, so much so most tracks were just planted in a giant field and that was it. Again, I think CM probably have greater freedom to populate stages with more varied and dense scenery, so stages won't just look like lines carved out in some vast expanse.
Alternatively, take heart from
Modnation Racers' auto-populate feature, which was masterful at generating interesting and lively scenery. Not the same type of game, but it does prove that can be done.