Stating that the UI consistency issues are due to backwards compatibility is stretching the truth to the point of outright deception.
Backwards compatibility is not what stops Microsoft from updating the GUIs for their own components. It is not the reason that the components that have been updated failed to carry forward 100% of the features of the components they're replacing, necessitating the ongoing use of the legacy components. It is not the reason reams of GUIs have gone untouched for 20 years. It is not the reason Windows now has at least 4 versions(!) of Performance Monitor, all of which are broken in at least one glaring way. (More on that below.)
The believable reasons for the GUI inconsistencies I've heard are:
1) API churn, because of which the UI teams did not have sufficient time to work on features versus playing catchup.
2) Bad system GUI APIs that are very difficult for even in-house teams to work with.
3) Unwillingness to take ownership of legacy code, with literally noone left at Microsoft willing to touch things like the ODBC connections panel or the Component Services snapin. People prefer to add new things instead of fixing or removing old things because it's "easier".
Actually, let's just stop here for a second. That last point explains the performance monitor views. I mentioned at least 4 copies, written over decades, by different people. Each new team has steadfastly refused to touch (or remove!) the old code, but hasn't replaced the functionality of the old code, so now end-users need 4 different versions to get things done.
These versions are:
1) The performance tab in Task Manager. The only GUI-based view that exposes some metrics such as GPU temperatures. Only shows a small number of fixed metrics.
2) Resource Monitor, which is opened from Task Manager. The only GUI-based view that shows certain per-process metrics, such as the names of files being touched, or per-connection network stats. Has permanent UI issues that will never be fixed, such as not using the system number formatting in some places, making large metrics unreadable as they change faster than users can count the digits. Similarly, the graphs take forever to change their vertical axes, making them useless 90% of the time.
3) The Performance Monitor MMC snapin. Totally legacy, with un-resizable controls that cut off text. Nonetheless, it is absolutely essential because it provides the only live GUI view of 100% of the performance metrics available in the system. It is also the only way to record metrics and view recordings. It is the only GUI for creating metric logging that persists. Etc...
4) The various versions of Server Manager's performance views, which are so useless that I've literally never used it. Nobody can get their job done doing this, we're all still using RDP to connect to servers so that we can simultaneously launch the three tools above. Why RDP? Because 2 of the 3 above do not support remoting.
I could go on and on like this for hours about how bad just this one aspect of Windows is, let alone the hundreds of other GUIs that have been butchered by bad decision making and internal NIH syndrome leaving a trail of half-baked messes behind.
Wait... did I say 4 performance metrics GUIs? I meant 5, because there's also the new Windows Admin Center, which was clearly written by people that have never had to diagnose a performance issue on a server. It's very pretty and utterly useless, which means that: Nobody will use it, and it will be superseded by someone else's half-baked attempt in a few years, 100% guaranteed.