Then again, Reiji, Xiaomu, KOS-MOS, T-ELOS, and M.O.M.O. didn't have amnesia when they first entered the Endless Frontier. There's a little YMMV though, since the latter three are all androids, while the first two are known for dimension hopping, and as such, probably wouldn't be affected by this kind of scenario after doing it for so long.
On that note, why no Wodan Ymir? He could have at least appeared at the penultimate level and instead of some Sanger clones, they could just use both W-15 Wodan Ymir and body snatcher W-05 Gagun Laos. Sanger's got so many clones nowadays anyway.
Too much awesome/collateral damage, considering their fight broke through something meant to withstand an alien apocalypse. Just imagine what THREE of them would do.