If it’s government money it makes sense to save $10m here to apply it there (in theory). But when it’s Boeing’s money it makes no sense at all. Boeing isn’t going to take the $10m they’re not spending and put guardrails on dangerous highways.
If unlimited safety improvements are required without regard to cost, it could drive the cost of the aircraft so high that airlines can't afford to upgrade, so they keep their older fleet in service for longer. Which works for a while, but airframes do age and need to be retired.
Boeing has an issue in their planes that the A350 doesn't have. So if the price of 787's goes up because it needs to go from unsafe to safe, then airlines can buy A350s instead.
In this case, the problem is with the engine, not with the plane, and the alternative engine is already approved and in service.
Mostly people don't do safety comparisons of different planes when booking trips overseas and, TBH, pushing that sort of investigation off on consumers creates huge inefficiencies in our economy. It's simply better for the government to hold all options to a certain safety level then try and rely on market forces to balance it out.