The philosophy in aerospace is more to build a better engine rather than to have more engines, and this extends to every aspect of aircraft design. Engines are already built to contain catastrophic failure, and the planes themselves remain functional for all but the most extreme situations. We're at the point where essentially every lost aircraft is a compound failure, with significant human factors contributing to the event. It's likely that we're on the pareto front of what engineering can reasonably accomplish, and the only gains in safety either barely nudge the needle of what we would notice (better materials, say), or difficult for the market to accept (removing pilots altogether).

Aerospace RND has been looking into hybrid propulsion systems for a long time. If there's one thing they aren't shy about pushing, it's the ability to go higher, faster, more efficiently. Such systems aren't used because they aren't yet good enough.