I think @TabbyBoy touches on this point, as a big change is that timeliness is more of a consideration, including with the flipping / arbitrage.
Sitting on a retired set and expecting it to go up forever is not realistic - if you're sitting on gold, just sell into hype and reap a comfortable profit rather than getting greedy. Keep that capital flowing with a positive margin and you're in a good spot.
There are a ton of examples within the past few years for sets that rose modestly, then dropped off again. Parabolic was mentioned. If a set is high and there's a good margin, just take the money and run, even if you're not on the top of the curve. No way to know that.
Things don't just accrue into skyhigh oblivion, especially when remakes, new huge lego sets, fakes, all provide ample competition with each other. There are more sets than ever to chose from, both new and old, and that's not changing. I don't think that's a sign of doom - making lego more available than ever is a good thing for its popularity, but devaluing brand via over-expansion is a concern too. Lego is a toy, and most lego is seen and bought as toys, which do not make good investments whatsoever. You probably shouldn't be hoarding friends, city, uniconic star wars sets, for RRP... some of the pics I've seen, those scare me.
Not all lego goes up or sells. Some things are solely parting out packs for bricklink stores if you get it on sale. And that's only if you don't value your free time and enjoy scoops of pennies and nickles.
There are so many lego sets now, you really have to chose wisely, or for good sales. I buy basically nothing at RRP, and only buy a few times a year when the deals or pickin's feel right. Stack points, wait for deals and sales, whatever.
Oops, don't mind me, just accidentally authoring misplaced speculation bubble posts ?