The last time Democrats held on to the white house for more than 8 consecutive years was WW2. They should get 4 more years of Biden, but the odds are that after that we will see a Republican president.
3 of the 4 justices under age 60 are Republican appointees,
The average age a justice leaves the court is 81, so it's likely we won't see any voluntary vacancies out of the younger justices.
That means it would be reasonable for 2/3 of Thomas/Alito/Roberts to make it through 2 more presidential terms. They can wait out 4 more years of Biden and even another 4 year Democrat term (which would break a long time trend) before retiring.
So yes, I believe it is very likely they maintain at worst a 5-4 majority for multiple decades.
Our best hope, and you are obviously more optimistic than I am, is a historical run of Democrat presidents that outlasts all three of the aforementioned justices assuming no other early vacancies.
All that said, we were this close to getting that very scenario had Hillary won in 2016. 🤷♂️. A few thousand votes in a swing state, and Mitch determined a third of the court for a 20-30 year run.
Republicans rig the game better than democrats