I'd say _that_ is a hell of an assumption. The travel industry depends hugely on efficient PPC advertising with companies at any serious size.
This has been true of all three travel startups I've worked at. AirBnb may well have great brand recognition but I'd be amazed if they can acquire customers for zero.
I can definitely accept that Rails is now somewhat old hat.
And agreed that the Rails 2 to 3 jump was (and still is for many codebases) a tricky and difficult path.
However, I'd argue that doing your 'startup' in [Rails/COBOL/PHP/Logo/Java] is probably a decent idea if you have good engineers who can build something stable relatively quickly. Technology is _rarely_ the problem in any given startup.
If scalability and speed is a problem, congratulations, you're a success.
Rails is still good at giving you the tools to build decent CRUD-ish apps pretty fast and deployment is thankfully a solved problem.
Rails is not the new hotness, but it's still great at getting prototypes out the door and can scale you a long way. I think I'm cool with that.
> our front end has gone from Prototype to jQuery to Coffeescript to Angular to React with major productivity improvements each time
Also rewriting your front-end four times doesn't seem that productive.