Can we assume those are models no longer offered?
I've seen plenty of the low end ~78% AFUE Burnham P20x series (now set up to run 82%) with 25+ years on 'em (not that I'd go out of my way to buy one.) I even scrapped one fully functioning with only ~15 years of service on it due to gross oversizing, when I got around to dealing with the system at the house where I'm currently living. The thermocouple on the standing pilot needed replacing at about age 12 (no bfd- expected, in fact), and the automatic flue damper developed an intermittency due to a cold solder on the printed circuit board at about the same time, which I repaired by reflowing the solder rather than replacing the unit. The basic boiler underneath had no issues, looked clean, burned cleanly. I didn't test the combustion efficincy, but it was probably still hitting around 80%. The ES2 & ESC series are probably not that different underneath the fancier controls and new tin, but I don't know that for sure. The 3-section Burnham PVG (same output & efficiency specs as the ESC) in my nieces place has about 5 years on it now (running ~130-135F return water) with no detectable signs of problems. I'd expect something in the controls or maybe the blower to maybe get funky by age 20-25, but those are component-swap repairs.