I'm not sure you're considering the fact that the web ecosystem itself is in competition with other platforms. The counter to your "If proprietary extensions get used" hypothetical is that if they don't get used, the world moves on, getting the job done with fully proprietary and/or platform-specific solutions -- Windows applications, Android apps, etc.
People don't sit back, put their needs on hold, and wait for the web platform to develop and implement new standards. They use available tools. Would you advise them to leave the web ecosystem entirely? Or use the lesser evil of extensions platforms, thereby solving their urgent problems and indirectly providing long-term direction to the web platform's evolution?
People don't sit back, put their needs on hold, and wait for the web platform to develop and implement new standards. They use available tools. Would you advise them to leave the web ecosystem entirely? Or use the lesser evil of extensions platforms, thereby solving their urgent problems and indirectly providing long-term direction to the web platform's evolution?