Earlier quoted context omitted.
(I used to work on the webserver for Google Search. Left Google several years ago, have no more vested interest in them.) The reason for the browser-sniffing in search is a combination of ease of testing + latency reduction. By supporting a known set of browsers, you can write a test plan for each and shard it out among contractors (or ideally, automate it with a Selenium config). Manual tests run on every release (2…
I was just going to ask why search should be browser-specific, until I read your last paragraph.
And finally, why not load the necessary polyfills and fixes on demand? When the old/slow browsers need time to get ready, a few hundred milliseconds more or less don't matter.