How long should Omega watches from the 1990's and 2000's last

It hasn't changed.

Yes.

Well we have watches that were supposedly worn by Mary Queen of Scotts and QEI. Not to mention the other high profile renaissance kings and queens. My friend and mentor (who I surmises was actually a metallurgist.) Said that the modern alloys should be stable for at least 500 years if properly serviced.

On the other hand I have boxes of scrapped movements. Strange to think of that these were once shiny new watches. So I agree with @Archer that it becomes an issue of parts availability.

One also has to realize that watches are produced in incomprehensible numbers. The term millions is often used. Yet when one thinks about it there is one watch for every person on the planet. (And a cell phone mobile too.) that means there are now a minimum of over 8 billion watches in existence and just as many mobile phones. (and companies have trillions in market cap.)

Back in the day, when say the pop of the US was 50 to 60 million. (I think it hit 100 million at in the 1960s.) There were enough watches for every one to have a new one every 2 to 5 years. Even if we have a few companies producing 100,000 watches a year, they will produce a million in a decade. 50 or 60 companies producing and well, the numbers are exponential. A lot of the reason why the industry collapsed in the 1970s. This would be a subject my friend and mentor would go on about.

Here is a what is left of a fusee watch. stored in a plastic movement holder.
This watch is probably over 200 years old. Such watches where also mass produced in the millions (or I would not have these parts.) Most likely the rest of it wound up in someone's steampunk art collage, or remains in someone's project box and auctioned off to sit in another project box.

-j