Will the currently released modulars still gain value after retirement, or have enough people caught on to what has happened in the past? I guess what I am asking is will the Palace Cinema appreciate like Fire Brigade or Green Grocer? I feel like enough people should have caught on to this trend, that modulars won't appreciate in value nearly as much as the old ones have, but will they still appreciate enough that a profit is made if you buy them and keep them Mint in Box?

Very few people looked at lego sets as a speculative investment play when GG retired (in 2010 (!)), which is why few copies were saved and prices for it are high now---demand is reasonable, but supply is low.  GG also sat for a long time at 20% off on the shop-at-home website in Oct of that year, showing that demand was moderate for it, even then.  2018 might as well be a different world: more copies of modulars are being produced, many are being horded, and they don't last for weeks on sale.   Depending on your time line and buy-in price, any set can successfully appreciate. I think the "buy it, forget about it, then cash it in" days for modulars are long past, however.  Tread cautiously.

I think the performance of PS and PC on retiremet has already answered this question. If and when lego re-release one of the earlier modulars, we´ll know a lot more.