I agree that in theory macOS 14 could drop support for all Intel Macs, but based on recent and past patterns I think that is way too soon.

Looking at the measure of "time after a model was discontinued that it is supported by the latest macOS" (public releases only, not counting betas):

1. During the PowerPC to Intel transition, the minimum for mainstream models was three years for the Late 2005 PowerMac G5, discontinued August 2006 and unable to run Snow Leopard in August 2009. The 2005 Xserve G5 was worse but was rare enough not to matter.

2. From macOS 10.12 Sierra to macOS 12 Monterey the minimum was at least five years with one exception: the Mid 2012 13-inch MacBook Pro got four years, but it was sold for longer than most models.

3. macOS 13 Ventura's minimum was slightly less than three years for one model (Late 2013 Mac Pro), just over three years for another (2017 MacBook Air), the rest were at least four years.

If we assume Apple's minimum for Macs is about three years after the model was discontinued (dipping slightly under three years for long lived models) then the Late 2019 Mac Pro should get at least three more versions (macOS 14, 15 and 16) but there is the question of which other Intel Macs come along for the ride. I'll refrain from going into a lot of detail, but for macOS 14 and a three year minimum, this includes some 2017 models (low end iMac, iMac Pro).

If macOS 14 dropped all Intel Macs then its minimum for this measure would be less than six months. We have seen that in other Apple product lines (e.g. iPod Touch) but never for Macs since the introduction of Mac OS X.

Why three years? It happens to match AppleCare coverage for Macs, and adding two years of security updates we get five years, which matches the minimum hardware servicing period from when a model was discontinued.

Rosetta 2 is a harder question: assuming no licensing constraints, it is probably a year-by-year decision when to drop it based on the installed base of "important enough" Intel applications still being used on Apple Silicon Macs. It should be there at least up to the last macOS version which supports Intel Macs. After that come the extra incentives of reducing the OS footprint and Apple's development and testing burden by being able to eliminate all the Intel code in the OS if Rosetta 2 is removed.

Waiting for WWDC to get a clearer picture.