The big pressure started when Microsoft dropped NPAPI altogether. See http://web.archive.org/web/20071016233843/http://www.meer.ne... (September 4, 2001).
Fast forward to today. I'm told that the next Android release, Ice Cream Sandwich, will drop NPAPI support -- this time in favor of Pepper. ActiveX, ActiveG.
Vertical lock-in from plugin to browser to OS to hardware. The 90s -- if not the 80s pre-commodity-PC -- are back.