Think "an interactive visualisation for choosing a seat when buying a concert ticket." That's not a "traditional" webpage, should work on whatever device the user is currently on (so native is unsuitable), and shouldn't require the user to install anything.

People clearly want web pages to be exactly this; we're on iteration #3 of the idea now. Iterations 1 and 2 were Java Applets and Flash, not necessarily in that order, and they were massively popular despite their shortcomings. Clearly, if we somehow deprecated the web as it is now (which is an utterly ridiculous idea, which makes me really curious what comments like yours are exactly trying to achieve here), as long as we keep it an open standard, users will choose browsers which offer this capability in some form. We should appreciate that the current iteration is an open standard which anybody is free to implement, instead of some proprietary blob that you need to license from Oracle or Adobe.