Tritium vials are so bright it probably won't be until they dim to a quarter of their new brightness in about 25 years (assuming the phosphorent material holds up as well as the tritium) that you'll even begin to think about replacement. 25 years from now ... who knows if replacement tritium vials will be available. There is a move away from tritium to superluminova going on mainly because although the tritium is not dangerous to the watch wearer, there are hazards involved in manufacture, and some concern about eventual disposal, especially if tritium paint and vials were to become very common. 25 years from now tritium might be banned in every country.
At worst, it'll certainly be possible to replace with superluminova dial and hands, or whatever kind of luminous paint is state-of-the-art 25 years from now.
If it's a quartz watch figure on replacing the movement sometime, too; 25 years is a good lifespan for most quartz movements.
Although a mechanical watch movement can last for generations with regular servicing, that servicing costs a lot more than replacing a quartz movement whenever it wears out.
Most of the watches with tritium vials seem to be in a price range where it'll make more sense to replace the whole watch than to replace movement and vials (if not the complete dial and hands) -- the cases are not that expensive.
To sum up, if you're looking for a watch to hand on to your grandchildren: buy a high-grade mechanical, expect to pay plenty over the decades for servicing, and you'll probably be better off with superluminova than with tritium.