The heat pump is likely to kick the crap out of any 20 year old boiler from cost effective perspective. Financial balance point is probably below zero. Did he run opcost for you?
Locking out at 35 for any logical reason, or legacy bias on the installers part? Maybe he's stuck thinking all heat pumps are the same as ones he installed in the 80's. Your equipment is like comparing a current notebook computer with a typewriter.
Disable the lockout. Find out how low it can go before register temps start getting cool. Then decide if you want to lockout somewhere above that.
Did you install a humidifier? Media filter? If so, you'll get significantly more comfort running the pump.