So if you want a longer career is better to stay playing in southern hemisphere
I mean, purely theoretical Calendar years yes: particularly for the right type of player in terms of position played, body type, overall skillset & early developer, you could see them playing 20 or more Calendar years of pro rugby if you stay out of Europe: with a chunk of that being made up of Japan & MLR towards the end of your career with maybe some Sevens towards the start.
But that definitely doesn't take into account either (a) amount of rugby you actually get to play, or (b) the amount of money you're getting paid to do so, both of which will be major motivating factors for most individuals who WANT to play rugby for a career. For your average, shall we say, "non-elite" player, the 8-10 seasons you would get from 4 to 6 years of European Rugby after 3 to 4 seasons in Super Rugby (Aus in particular, as NZ's NPC alleviates a lot of the lack of "professional" gametime arguments that are relevant here) is going to end up giving you significantly more total matches played and more money earned than the 10-14 seasons grinding out another 3 to 6 years in Aus then having a few semi-retirement years in Japan/MLR.
Indeed, there's arguably more of an opportunity to stay a part of a pro team for longer since, as more rotation is required in the squads to manage bodies, it's also much easier to carry an older player who you know who's body needs a lot more rest time: particular for your tight five and stand offs. Here there seems to be far less variability in expectation at pro level: you're either a Wallabies player who may need management to ensure they can play all 14/15 tests if required, you're a developing talent who can make some bench appearances when needed, or you're a mostly developed player who needs to be capable of showing up and playing every single game if required, but is not too valuable to not drop when a youngster breaks out or a Wallaby comes back from injury.
And of course, nothing stops you from going to Japan/MLR for that career extension after a European career either. 37 year old Greig Laidlaw played 340 games for Edinburgh, Gloucester, Clermont, Scotland & Scotland A across 14 seasons, moved to Japan and is about to begin either his 3rd or 4th season with a side there.