The thinking is that Russians should bleed and suffer economically and demographically is success enough, without Ukraine "winning" per se (that is misguided thinking)

Backing out paves way for full annexation of Ukraine in the future and a power shift in the Black Sea, so where is the success in that.

The only way I can rationalize this is, the West wants to establish a peace now to rebuild and prepare for a great war in 10-15 years. But then they'll risk their own men and women whereas now only Ukrainians suffer casualties. Everyone felt now is the right time to go all out but I guess not