A landlocked Ukraine becoming a defenceless failed state that's far easier prey for an inevitable round two, just as not resisting the annexation of Crimea led to the next round in the Donbass in April 2022 and a full invasion on February 24, 2022.

Also, the probability of Russia respecting a peace agreement is a very simple number: zero. Ask Georgia and Moldova.

Also, a long term frozen conflict with a border dispute makes Ukraine's accession to the EU impossible:

"The EU’s enlargement strategy, entitled ‘a credible enlargement perspective for the western Balkans’, states that prior to EU accession, the candidate countries should resolve their bilateral issues, that is to say they should resolve all their border disputes before the conclusion of negotiations. In light of the experience of Croatia, which was blocked by Slovenia during accession negotiations due to unresolved border issues, and the subsequently agreed arbitration process, which could not be implemented following Croatia’s exit from arbitration:

Does the Commission intend to prepare a procedure that would be applied in respect of candidate countries’ unresolved border disputes in order to prevent accession negotiations from being blocked and to ensure that such disputes are effectively resolved?"