Newer bikes may last for more miles, but you had better put those miles on in a hurry. New technology does not hold up over time, and it is not designed to be kept going over a long period of time. Old bikes can be kept going virtually forever, depending on whether parts are available, but newer bikes become too expensive to be worth fixing in a fairly short time. A carb will last a lifetime. A FI ECU, which costs more than a new carb, has a fairly short lifetime when measured in years, and will probably no longer be available if you need a new one 20 years from now. New bikes are designed to be disposable and recycleable. Rather than fix them after a certain point, you are just supposed to replace them. Honda has started setting the lifespan of their new bikes at 10 years, after that they will no longer work on them or sell you parts for them. There is a definite growing trend in this direction.
I won't spend more on a bike than I think it's worth, but if it's a good deal, I don't seem to have a problem buying bikes I don't need.
2002 Vulcan 750, 2013 Royal Enfield B5
2001 XT225, 2009 Genuine Stella
1980 Puch moped