It would make sense, since Apple would make more money out of every single customer buying an Ultra, but... yeah. I don't think that the company will see it that way.

It seems that the number of years an iPhone will be supported for depends more or less on the processor it's based on. Apple is supposed to be testing the newest iOS against the capabilities of each Axx chip and if performance on e.g. the A12 is acceptable, then that iPhone is supported.

There have been some exceptions here and there but that's the general idea. So what will most probably happen is an Ultra getting as many years of support as any other 2023 iPhone based on the A17, like the Pro for instance. Unless, of course, the Ultra is the only model getting the A17. It's all conjecture at this point.