It is not my position, but allowing for the maximum degree of good faith, I think what some people considered as backwards compatible was essentially switching out the 2014 PHB classes/species with later, improved versions from Tasha's and Xanathar's, plus a healthy smattering of errata for problem rules/spells/features. In other words, greater continuity between the 2014 rules and the 2024 rules.
WotC has not done that. They have revised just about every facet of the game, to the degree that, say, even the 2014 Champion Fighter and the 2024 Champion Fighter are two distinct things now. And so now, a player and/or a group has to make a distinct decision: 2024 rules or 2014 rules? And if the former, are 2014 options still available?