1. There isn't a low-level performance bottleneck within most web-dev code. It's almost all occurring at levels slightly above that(disk access, network bandwidth, database queries, horizontal scaling). The stuff that needs to eke out another 50% in an inner loop through optimizing cache access or careful use of hardware parallelism tends to be a single algorithm or batch process that can be developed independently of the larger application.
2. For eliminating whole categories of errors, there is a bounty of options out there that can claim to improve on C++ through better tools, type systems, and compiler technologies. Even Java, as maligned as it is, can constitute a credible step forward.
3. Good C++ developers aren't that common. There are plenty of junior developers who are eager and ignorant of the potential dangers, though - they will make a mess in any environment, but they'll make a bigger one when given power tools. (This is the argument for Go, and w/r to the case of low-pass-filtering junior devs it definitely makes sense.)