The 25th amendment was enacted in the immediate aftermath of the Kennedy assassination. It was intended to deal with a specific set of facts that the -- (edited)
impeachment provisions did not address. A president who is not physically capable of doing the job. The fear that if Kennedy had survived the shot to the head, but was in a coma, there was no way to remove him from office. He had not committed any "high crimes or misdemeanors" but could not fulfill his obligations. In fact, it mostly addresses temporary situations where there is incapacity and provides mechanisms for the President to resume office. You are suggesting that it was taking certain powers away from the impeachment provisions. Please provide authority for that because I have not seen any arguments to that effect. I'm not a litigator or the constitutional scholar, but when the plain meaning of the language puts provisions into conflict with each other (as I believe is the case in your interpretation) you have to look beyond the plain meaning into the history to try to reconcile them. The question of how broad the impeachment powers goes is a long debate that is not resolved. I found this article that lays the arguments out pretty well. I have seen more arguments to the effect of the scope of high crimes and misdemeanors under English law (a term of art under English law at the time adopted in the constitution) and how that was broader than legal criminality (including appointing people who were corrupt of did a poor job, etc.), however the issue continues to be debated.
here (constitution.congress.gov)
