Microsoft was trying to promote Silverlight for years and it fizzled just fine without anyone's help. While Silverlight has a bunch of notable technical advantages over HTML, it also has a number of disadvantages (PopeDai summarizes very eloquently). The disadvantages proved fatal for the browser plugin's prospects.
where and when it died had a lot to do with when and where Microsoft started to kill it.
as a new tech it needed time for corporate customers to fell safe adopting it, just when that was starting to happen Microsoft started saying things and doing things that made everyone suspect that Microsoft did not want to put any more into the tech and wanted to push developers in other directions. that is what killed it.
if MS had not delivered the mixed messages and delivered the updates that never came out....
and yes, at one point they did state that they had an update planned for Mac OS that would hook into the Mac to allow better Mac / Silverlight apps, that would have been in the last version they did on Mac but the features got cut due to them halting the work.
Silverlight would never have been a "replacement for html" it could have been an "option" to use when you want to do things that html is not the right tooling for.
folks can ignore this all day long and it still is true that there are things that could be done better by not using html and all it's bits of cruft.