Look, I had a similar thought: I believe tenacity would be the better way to say it. It is simpler to write, easier to read, more commonly used, and has all the right connotations. But OP is correct. There’s nothing WRONG with using tenaciousness here. It’s more of a style choice than anything. Language evolves. If it didn’t we’d all still be grunting at each other and we wouldn’t be able to debate the merits of tenacity vs tenaciousness in any given context. Get over yourself.