Old version support has a cost for the company supporting it. I guess it shows to the maturity of the company when they do. It's not easy to pull off.
As a dev on a mid-size project, I'd say that those UI changes often have two sides, with people who will also complain "why can't you just fix/change X, it's such a small change, but so annoying". Everyone wants their issue fixed, but not the issues of others.
Our project went from having two versions (stable and old-version), to just one, because the community of users had split into two, with much fewer developers working on fixing the latest version (because their company mostly supported users on the old version, not deploying new projects on the latest version, very understandably risk-averse people).
Removing the old-version made a few developers angry, but 80% of them rallied and now the project is much more sustainable (i.e. we're not big enough to afford the split). Eventually we introduced a paid version of the old-version supported for 6 months, with a stretch goal to extend to 12 months if people supported it. Only a handful are paying for it. Others are now happy on the latest, which became more robust as more people are using it.
UI is surface area. Having 2 UIs vastly increases the surface area of the application, which means tons of testing. It also means all of your developers have to dedicate resources to remembering 2 or 3 versions of the UI, and to an extent the history of those changes.
There are also degrees to which it's more difficult to memorize two loosely related things than it is to memorize two completely separate ones, because your brain keeps trying to treat them as similar even when they are not.
As a dev on a mid-size project, I'd say that those UI changes often have two sides, with people who will also complain "why can't you just fix/change X, it's such a small change, but so annoying". Everyone wants their issue fixed, but not the issues of others.
Our project went from having two versions (stable and old-version), to just one, because the community of users had split into two, with much fewer developers working on fixing the latest version (because their company mostly supported users on the old version, not deploying new projects on the latest version, very understandably risk-averse people).
Removing the old-version made a few developers angry, but 80% of them rallied and now the project is much more sustainable (i.e. we're not big enough to afford the split). Eventually we introduced a paid version of the old-version supported for 6 months, with a stretch goal to extend to 12 months if people supported it. Only a handful are paying for it. Others are now happy on the latest, which became more robust as more people are using it.