Peoples tastes always change with time, new games get released, old favorites get played less, space becomes limited, cash flow issues, etc etc etc.
It all comes down to how much space you have & how much money you want to put into your collection. Your "ideal" collection is a result of those two factors. For me it's not so much about owning specific titles anymore, but having the right amount from each era to have an ideal balanced collection. That plan gives you some leeway to swap titles down the road, but still be happy with your collection.
I have room for 8 pins. My ideal collection would be...
2 modern pins (2000-present)
2 DMD classics (1992-1998)
2 System 11's (1986-1990)
2 Bally/Williams/Stern (1980-1983)
I'm getting there eventually, just takes time.