She didn't institute "growing new crew" because - given the increased lifespans of humans in the 24th century - SHE firmly intended to still be Captain when they got back. Janeway was 40-something when the mission started; add 70 years to that, and she'd still be well within human lifespan. So who needs a new generation of crew when Janeway intends to command basically forever?
Ah, but remember, human life expectancy in the 24th century seems to be up to 150 (on average) - which is not that large given their technological capabilities and humans in real life already reached 120 - and setting a standard for people would not be that difficult since longer life expectancy seems to be correlating with lower calorie intake. Throw meditation into the mix, and you can probably surpass 120.
Though, humans in real life are effectively poisoning their bodies with all sorts of chemicals that are used in agriculture (outdated as it is for our day and age - with too large of a footprint seeing how vertical farming employing hydroponics alone could severely reduce our footprint and produce foods that are rich in nutrition without using any chemicals, pesticides or gmo).
Setting what I said aside... you are correct that Janeway seemed to be in her 40-ies when Voyager departed and barring any premature death, could (canon-wise) still be in command 70 years later - and yet, in Endgame, she (and the rest of the crew) were portrayed as ageing geriatrics some mere 26 years later.
Trek writers conveniently forgetting about the longer life expectancy.