There was a time when the creation of a new TLD was unthinkable, so it seemed safe to use a domain syntax internally that was never going to become a public TLD. Then the TLD money grab happened and what was assumed to be safely isolated wilderness was sold out from under everyone.

There never was such a time, and IANA never said "We won't make any more". TLDs have been continually created for at least the past two decades. Look up the history of e.g. .biz, .info, .museum, .aero, .mobi, .cat, .asia, all the various ccTLDs that are created as new countries form (.ss), etc. And none of that is even counting the new gTLD expansion round that kicked off in 2012.