If the bike came with 27" wheels and they are in good enough shape, whey would I change them. I even took a pair of wheels that had good 27" rims on it and re-spoked it with double butted stainless steel spokes.

As far as tire availability, it is good enough. I need an inexpensive tire for when I flip bikes and my lower performance bike, and a good tire for my current rides. They doesn't need to be 100 choices.

The Kenda K35 is inexpensive enough, available in gumwall so it looks right on a 70" or early 80's bike an rides well for a low to mid performance tire.

There are lots more choices for better performance tires. I can even get vintage looking skinwall tires. I will be ordering Pasela's soon.

If things came together where I didn't have 27" rims, and had to buy new rims, and the bike and calipers would fit 700c, then I would go that way.

My LeTour, Traveler and Continental all have 27" wheels. I don't have a bike with 700c wheels.