Sheltron's tradeoff is that as the expansion goes on it will get strictly better at blocking most damage than other on-demand cooldowns, because block strength will likely get to the point where it mitigates about 25%. There's not going to be any content that straight out requires you to be able to ward off an autocrit, even shit like Optimized Blade Dance in O12 and Tera Slash in Shin Ex will still be fine, the same way magical tankbusters don't instagib non-DRKs. PLD is allowed to be slightly worse in one aspect of tanking that rarely actually surfaces, especially with how overwhelmingly good it is by almost any other metric.