Would you buy a tube amp if you were unable to use vintage tubes in it ?

How long a tube lasts, it believe, is primarily that of tube design.

So for instance a 6550 tube will typically have a ~3,000 hour life.

On the other hand a 300B will have an expected live of 40,000 hours.

Many small tubes share very long life. 

I just recently learned about the 300B. I decided to get my self a backup set of Takatsuki 300B tubes (Japanese) in case their price went up considerably because of tarrifs. At $2K for the pair, a sizable increase in price would be a lot of money. Then I looked up the average life... 40,000 hours. Maybe I didn’t need the backup set... I’m over 70. Oh well. 

Secondary factors are how "hard" they are run in the design and if power tubes, how hard the amp is run (as in how high is the volume). 

I would say there is no substitute for great design. In simple inexpensive amps perhaps the tube can get you somewhere better and more sophisticated. But for carefully designed high end amps, the tubes are chosen for availability and general character and then the rest of the component is designed to achieve a very specific sound. The goal is the specific sound characteristics… and all the components are chosen to achieve it. I am certain this is how companies like VAC, Conrad Johnson, and Audio Resesrch create their components. I have heard an interview here and there that supports this as well as worked with high end audio engineers. So you can change their sound… but typically the sound quality is balanced on a knife edge between a number of characteristics and it’s only likely to be different but unlikely to be more sophisticated.