Will Gen 4 end the era of the non-legendary dragon generalist?

To address both points:

While Dialga and Palkia are likely to join Rayquaza in the top echelon of dragon type attackers, the answer is the same as most of the Gen 4 speculation; it depends on their moves. While DT/OR Dialga fits between Salamence and Rayquaza for dragon type DPS a DB/DC one is below a few variants of Dragonite still.

I think part of what makes dragon type generalists popular is that dragon type moves are resisted by very few, relatively uncommon types. In my area anyway I don't see a whole lot of fairies on gym defense despite resisting the four-armed fister and steel types are usually kept away for that reason.

My mind is blanking right now so I could be forgetting others but the only Pokemon Gen 4 introduces that raises my eyebrow about the defence scene is Togekiss. While it slows down Machamp *and* dragon type generalists, I don't think a single Pokemon introduced to the gym scene will be enough to heavily shake things up. At most, people's gym teams will now pack a Raikou if they don't already.

Dragonite will always be a solid attacker. It may not be THE BEST at its roles, but the relative availability of candy (especially if you played on CD) makes it a more appealing choice than the legendary dragons.

Rhyperior's utility is unknown without knowledge of its potential movesets. Steel is resisted by more types than Dragon, making it less of a generalist, although MM Metagross will definitely have uses.

Garchomp also has a double weakness to Ice with no resistance to Machamp, so it brings nothing to the table defense-wise that Dragonite doesn't. Togekiss will probably be a better defender but Dragonite will still rightly have a place in gyms.

The 1.4x multiplier for type effectiveness has already lessened the impact of generalists across the board. Generalists are reserved for filler on raid teams unless there's some kind of weather boost in effect (e.g. Moltres). That said, it's hard for me to envision a situation where Dragonite has no use in this game.

Dragonite inflicts crazy neutral damage on gym defense with its DT/DC set. Only other mon that is that damaging and punishing on defense is Gardevoir because of its high-power defensive set, and Gardevoir doesn't have the bulk that Dragonite has.