Now at 40 the brand is a bona fide long runner that is almost self-sustainingly running on current relevance and goodwill as much as nostalgia, not too dissimilar from how Star Wars has been, and certainly faring better than brands like GI Joe that couldn't parlay the nostalgia into current relevance for any number of reasons. The collector market has grown considerably, and kids are a bit harder to rope into the brand and a bit less certain as a market. It makes sense to play the cards that are there, and I'm sure the team would enjoy the opportunity to go in new directions if that were more of an option. But the drive to maintain an evergreen core is almost certainly not their call alone, but part of the company's expectations for the handling of the brand.
All very well said, but I wanted to highlight this because it touches on probably the ground zero for all of this... Marvel Legends.
Super hero figures predated Marvel Legends but prior to that they were very kid oriented- limited articulation, simple or stylized sculpts, and action features.
Marvel Legends was ToyBiz experimenting with a line aimed at collectors first, with an emphasis on nostalgia.
No Marvel Legend reinvented the wheel. No new designs were achieved via that line. Everything was strictly about replicating the comic designs (and later tv show, movie, and game designs) as accurately as possible with an emphasis on articulation and accessories. No action figures to "compromise" the figure, and generally more paint apps then you'd usually fine in a kid-aimed line.
So already you can see the trends starting- forgoing new ideas to replicate existing designs, emphasis on accuracy, articulation, and paint over gimmicks, and pack in accessories/effect parts. It could all apply to Transformers Generations from Siege onward.
But my point is that this was the first time this had happened. ToyBiz's earlier Iron Man TAS line had gotten very inventive with snap on armour and cool new designs. That wasn't happening with their Marvel Legends line. It was strictly replicating the looks the characters were known for.
Marvel Legends ended up making the six inch scale the default collector scale, with companies like McFarlane and NECA sort of outliers in the seven inch space.
My point though, is that as other IPs and brands moved to become "compatible" with Marvel Legends (Star Wars TBS being the most notable example though far from the only one) the ML approach took hold. That's just the standard now... and I think Transformers Generations began shifting towards that fully in the Prime Wars Trilogy, but fully shifted in WfC.
Looking back though, I think ToyBiz really kicked it off with Marvel Legends. Which is, funnily enough, a Hasbro property now.