If you do cut the other trees down, maybe wait a few years untill they decline? Chrries and plums are good back up for a super harsh winter.
Either way, you need the most cold hardy cultivars with the most vigor...Chicago Hardy, MBVS, Hollier, etc. If you can keep them from dying back, eventually the will need much less winter protection. DO you intend to expand for a commercial venture, or is this a family food plot?