I look at the boilers at work, most near or over 100 years old, but like the 500 year old axe, had some new heads, and new handles, but not both at same time, so although no part 500 year old the axe is considered that old.
Every 10 years boilers need a major strip down and test, but we stopped using boilers for central heating many years ago, they heat the water, but not to boiling point, so if not pressurised they don't require the same maintance. But the hotter the water the smaller the radiators can be, and water heaters seem to be working at lower and lower tempertures, so in some cases to use the better water heater would require changing radiators, in some case with a fan assisted type so they can heat and cool.
It does seem oil water heaters last a lot longer than gas, and are also cheaper to run, but can cost twice as much to buy, at work the boilers are solid fuel, and main problem is finding coal, the overts designed to replace coal don't seem to work very well, we may in the future have same problem with liquid and gas fuels.
So really no answer to the question.