People always like the new shiny, and we have been blessed to have an avalanche of great games in the last decade or so.
MM will always be great, but will it always be the greatest? I sure hope not. There are so many great games coming out now that newer collectors probably can't fathom what it was like 15 years ago. We got one classic every couple of years if we were lucky.
Almost everything has improved pinball wise over the last decade or so.
You want better animations? Check. Deep rules? Sure. MORE flipper power; heck, here are airballs galore with 70v flippers (or whatever current Sterns are ). New never before seen mechs? We can start sprinkling those in more often.
Better sound? Better light shows? Co-op modes Are you not entertained?
About the only think I can currently knock Stern for is questionable repairability on Node boards, embarassingly loud ass power supply fans (I swear it sounded like my neighbor leaf blowing the first time my JP fan kicked on...) and not updating their menus since what feels like ~2001 (why not improve the user interface now that you have a better screen?).
None of that changes the fact I would take a collection of say 10 games released over the last decade over a collection of 10 games from the 90's any day of the week. It is good not to HAVE to choose that, but if you are a halfway decent player you will burn out on those 90's games way faster than the new ones. At least that will be most people's experience.