The boiler is 15 yo, it's just a conventional boiler but still in good shape and runs clean. Never had any issues with it, but there's really nothing that can go wrong with it.
Good point with the hybrid, One service call negates any efficiency gains.
If the WH were up against an exterior wall, an on-demand WH would make more sense because the venting would be easier but it's in the middle of the basement where the chimney is.
I'll try increasing the diameter of the vent and look for a stubby WH, maybe that will help.