Competition is a great way to drive innovation. What will motivate innovation on the web platform going forward? Will there be stagnation? Will another company emerge to push the web forward?
What will this mean for companies that have enjoyed all of the innovation on the web platform in recent years? How much longer will it take for bugs to get fixed?
This will not be a sudden shift. It will happen gradually.
That's what happened with Internet Explorer but the reason Microsoft was perfectly happy to stall the development of the web once it took over the browser market was because it didn't really have any incentive to use the web in the first place (it was better off if people just used desktop apps), and it only developed IE so that if the internet was going to take over it could be the one in the position to control it.
On the other hand, outside of Android, Google is extremely reliant on the web for its products on desktop and on chromebooks (there used to be chrome os apps and chrome browser apps but they have already been deprecated).
Google actually has an incentive to add new features to try to give PWA's feature parity with desktop apps, so rather than ceasing development, it may be more likely that they start just adding new features that aren't compatible with other browsers.
E.g. they could probably now go back and immediately undeprecate WebSQL and just say, "we don't care what firefox thinks now." If they announced that, they have enough market share that they could probably get people to start using it again and that would immediately break compatibility with firefox.
If they decide to add new amp integration or similar features, for example, that can give them more control of the Internet even if you are using those features through Edge rather than Chrome.
Seeing Google's goal as getting everyone on Chrome is missing the point: it's just as good from their perspective to simply have everyone using chromium based browsers.