In the far term as space becomes scarce we will need to find a solution to wasted B space, but that is several years out. Perhaps by that time routers will have so much memory and CPU as to make an extra ~4 million possible routes negligible.
The danger of /17 blocks in B space is limited to 64*256 more routes (16 k more, maximum).
Yes, you could arbitrarily say /17 is a fair border, and then people would complain about their /18s being unreachable. The 4 million number reflects 64 * 2^16 theoretical /24 routes - 64 * 256 current theoretical /16 routes = 4177920 routes. I haven't heard (yet) of people complaining about not being able to get /25 to /32 routes globally routable. Perhaps a somewhat less arbitrary limit corresponding to the smallest allocation made by ARIN would be in order. That would currently be 2^(20 - 16) * 64 * 256 - 64 * 256 = 245760 extra routes. Still a pretty highg number, but I imagine it would take several years to break up the existing Bs. Austin