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

The 40,000 hour figure comes from original WesternElectric 300B operations and does not necessarily apply to all 300Bs.  Some, such as Elrogs were notorious for quick deaths.  As for the new reissues, it is to early to tell, but at least they have a long five year guarantee.  Another issue is that some modern amps really push tubes very hard to be able to claim high output power.  I would personally prefer an amp that runs the amp very conservatively.  Another issue is that a lot of American power grids deliver power at 120 volts or more and this can be hard on tubes.  I know tube people that employ step down transformers, one to as low as 100 volts.

How can I know if my amp is hard on tubes or not ? And what about 12AX7 and 12AU7 signal tubes, can the amp be hard on them too ?

I use PS Audio regenerator that puts out 120 volts. Well, I could, I suppose, put a step down transformer between the regenerator and the amp. How would the amp sound with 100 volts input, I wonder ?

I don’t think you need to go so low, 115 volts would be within any specified operating voltage and it would be significantly gentler on tubes.  Too low and performance may suffer and might actually be a problem if your amp has voltage regulators.  The person I know who set voltage at 100, knows what he is doing and can adjust other operating parameters of his gear.  Some amps do run small signal tubes hard too, but that is not usually a big concern.  There are some who don’t use these tubes as drivers for the 300B because they may not have enough grunt (or may be pushed a little hard to deliver sufficient output), but many amps do use these tubes as drivers.  The subject is complicated, as some amps use interstage transformers which reduce the need for grunt from the driver.  Hence, it is hard to generalize.

In order to determine life expectancy good to know plate voltages, ask amp manufacturer or measure plate voltages. Soft start circuits will also extend life, in rush current can be hard on tubes.