To be totally fair, Brendan Eich has stated that he was well aware that Lars Bak et al were working on V8 years before Chrome was announced (and Bak's involvement implied the implementation direction that V8 was likely to head towards). I'm sure that knowledge was known elsewhere as well.

The problem with this discussion is inherent in any counterfactual history, as you point out, and the common issue of what appears to be revolutionary to people outside a domain vs merely implementing things people have discussed for years, as the same "revolution" appears to people inside that domain.

The reality is that circa-2008 JS engines were fairly rudimentary, including (or especially) V8, and your average JIT compiler writer at the time would have been less impressed and more compelled to question why this "revolution" hadn't happened years earlier (and several did ask exactly that).

All that said, "Chrome put pressure on other vendors" has been extremely important to the evolution of JS performance, especially with Crankshaft -- I think it's extremely likely that shipping Firefox would still have a tracing JIT today and would be just starting to move past it without Crankshaft having existed -- but Chrome in that slot vs the other major browsers is really not that meaningful a distinction either, as without SpiderMonkey existing, it's likely that V8's major strides forward would have stopped with Crankshaft in 2010. Performance would have continued to improve in either (especially with JSC making big improvements), but I don't think we'd have seen the major architectural changes as often as we have without that pressure. Competition is great.