I ride 27" on a couple of my bikes that are built from older parts. So far as switching to 700c, I figure that someday I'll need new tires and will discover that 27" are no longer available, then I'll build new wheels. Or if something really bad happens to one of the wheels. But I see no reason to worry until that actually happens. "Premature optimization is the root of all evil." -- Donald Knuth
Continental Ultra Sport and Vittoria Zaffiro have been available in recent years. Of course everything is in shortage right now, but these seem to be available in 27x1-1/8 and 27x1-1/4
https://www.biketiresdirect.com/prod...ffiro-v-27inch