Google may be motivated to attempt things like Dart and NaCl which help drive forward their own products first and foremost, but they have a consistent reputation of making their web-advancing client-side technologies open source too (Dart, NaCl, WebM, SPDY, Chromium, Webkit, etc are all open source). They also have Mozilla, Opera and Apple pushing forwards as well (though I'd argue that IE9/10 isn't putting much competitive pressure on anyone).
If Chrome is abandoned by Google you're free to fork Chromium, acquire licenses for it's restricted plugins and it'll only slow the web's march forward, not actively hinder it. It's entirely possible for another group to carry the Chromium torch, and Google/Chrome has never yet intentionally damaged the quality of standards-compliant competing web products (though they don't offer quirks-mode fixes for crap tailored only to IE).
IE9 and IE10 certainly are putting competitive pressure on others (including Mozilla, my employer). For example, at the time of release IE9's vector graphics performance was far ahead (we're talking on the order of 5x) of any other browser, including Chrome and Firefox, and IE also led the way with background compilation of JavaScript (a very important feature, arguably more important to users than raw JS performance).
If Chrome is abandoned by Google you're free to fork Chromium, acquire licenses for it's restricted plugins and it'll only slow the web's march forward, not actively hinder it. It's entirely possible for another group to carry the Chromium torch, and Google/Chrome has never yet intentionally damaged the quality of standards-compliant competing web products (though they don't offer quirks-mode fixes for crap tailored only to IE).