There's 650b conversions of old road bikes, new 650b bikes as a randonneuring style, modern disc brake bikes that use 700c or 650b in an attempt to be all rounder bikes, and then there's 27.5. So to answer the OP's question, probably the first in that list is the only thing that would be a miss - if you're missing out improving an old road bike.
For decent, affordable 26" road slicks, Primo Comet 26x1.5, about 38mm, sometimes can be found for $35. 1.5" being either too skinny, too fat, or just right, depending.