"Way less justification" is wild
Natdex brings back two generational mechanics, one of which was incredibly popular, and cut mons and movepools that bring viability back to a lot of mons. As a result, the landscape of Natdex is drastically different from regular OU, with mons barely viable/non-existent in regular OU sitting at the top in Natdex, like Tapu Lele, Ferrothorn, Heatran, Tornadus Therian, and more.
Meanwhile, a Teraless ladder would remove a defining generational and split the playerbase, for what? A meta that's still dominated by mostly the same mons? Great Tusk is still going to be an incredibly splashable mon whether or not it can turn Water type. Iron Valiant is still going to outspeed 98% of the metagame whether or not it is secretly a Steel type. The only mons that would really take a hit are Kingambit, Garganacl, Serperior, and debatably Volcarona, but they would all still be semi-viable to good. Heck, it's not even a very popular idea: Natdex brings in around 45% of games played compared to OU (650,000 to 1,400,000). That's 17% more than the percentage on the survey which voted for Tera's ban: 28%. And that's assuming everyone who voted ban would proportionally represent all of OU's playerbase, as well as all migrate over to the Teraless ladder.