You'd need to know what the burn rate is composed of: their engineering organisation might be less expensive than the marketing budget in a single major market.

If they cut ad spend and raise costs, then their growth story stops and other competitors have a window to move.

The go-to reading (at least a few years ago) for handling the difficulties of burn rate at a high-growth company is 'Mastering the Rockefeller Habits' by Verne Harnish, the chapter on cashflow. I think 'The hard thing about hard things' also talks about how this situation can kill your company if you're unlucky or unprepared

Being a glorified taxi dispatcher probably isn't Uber's idea of a successful endgame. They desperately want to be a big player in the future of autonomous transportation and are clearly willing to bet the success of their taxi dispatching company to do so. Cutting their burn rate to a skeleton crew means giving up on that ambition and spending the rest of their 'life' as a reasonably successful company, with perhaps some modest profits, dispatching taxis via a phone app, and neither they nor their investors seem to want that.