They were held back in the context that it wasn't worth putting r&d into maximizing the throughout of an ssd when your primary markets all use a 5400rpm mechanical drive.
Not in the sense that they were explicitly developing game's to load slowly.
everything you choose to do or not do when developing a game is a choice, not spending resources on multi threaded loading that could utilize ssd and fast cpu would've resulted in more time to improve other aspects of those games.
now it's worth it because the benefits will be felt by a much larger portion of the playerbase, but again you get this at the cost of something else they could've worked on. Every feature is a trade.
Be thankful that both next gen consoles have hardware solutions to io bottlenecks because it might mean significantly faster adoption of rtx io and directstorage in the pc space, even if the average gaming pc can't use them very well.