No, it can't fill the niche, but it definitely lessens the novelty of games coming to Switch. Seeing, say, NieR: Automata portably isn't going to be the same to some when, right now, you can buy a device that'll likely run it at twice the performance and resolution. That + Steam Deck games aren't exclusive ports that you have to pay full price for, as are the majority of third-party titles. It's just simply not as exciting seeing these big PS4 titles releasing on Switch as it was back when DOOM was this massive thing.
As for "a good majority of Switch owners are fine with 3rd parties." Yeah, I'd say they're fine with them... but looking at the actual sales, they're not even close to the 1st-party stuff when it comes to sales and - in turn - popularity. Going by Wikipedia:
https://en.wikipedia.org/wiki/List_of_best-selling_Nintendo_Switch_video_games there is a single third-party title in the Top 20 (a console-exclusive Monster Hunter game, so not a late port), and only 6 total in the Top 50 (excluding Nintendo collabs like M+R and HW:AoC). That's... not much. Especially when you consider how, of the few that are there, there's Minecraft, Among Us (massive casual titles) and Octopath Traveler (an exclusive published by Nintendo themselves in the West).
So, third parties definitely have an audience on Switch. I should know - I buy way too many of them, and there are some that I'd genuinely love to see on Switch (
cough Fallout 3/NV
cough). But, they are not as popular, and will likely never be as popular, as first-party titles, or third-party titles on Xbox/Playstation. As such, a Direct focusing on them is going to generate more disappointment overall than a regular, or 'regular' Mini.
Cool, no worries