I was totally okay with this performance.
1) Like koregaboku just said, it makes the original versions sound good in comparison, reinforcing H!P's relatively elite status as being an idol troupe with leads who can really sing
2) I don't hate Oomorin's voice. Furthermore, as Celedam alluded to, Seiko Oomori is a kind of uber-H!P fan (Amped
himself posted about one of her prior partnerships with Sayu), and she's enough of an honorary member to have her own page on the
H!P Wiki.
3) I really enjoyed seeing Sayu and Maro performing again, and not trying to hog the spotlight. Neither one of them were anywhere near top tier singers while in H!P, their talents more lying in their cuteness and creativity, so for me it works out for that they didn't pick a return stage where they'd be upstaged.
4) I tend to like covers in general, and I love the idea of H!P songs being covered, even if it's at a less-than-excellent level of performance. Seems to me it could at least encourage others to do a better job, which leads to the final point:
5) Tsoonk getting performance royalties >>> all other considerations