Let me state right off the bat that I am no boiler expert. But in discussing this with my father, who was in the air conditioning business for some 50 years before retiring, let me throw in my two cents.
Dad is of the opinion that many of the old air conditioners, refrigerators, etc. given the materials available at the time, were better designed than the stuff today. (We have a 1948 refrigerator that still works fine and keeps stuff cold - don't ask me how Dad keeps it going - truth be told, and industrial archeologist would love to get into Dad's garage).
The difference now, is that the materials today are so much more advanced, that "better designs" are now possible. I know the new air conditioners are much more efficient. My brother is in the air conditioning business now, so this gets discussed.
But everyone, in our family at least, is in agreement; the older stuff was much easier to maintain and repair and held up a lot longer. I am certain a lot of engineers would weigh in on this and disagree - but those same engineers have not stood on a flat top tarred roof in south Louisiana or south Texas when its 98+ at 100% humidity, no breeze and who knows what the temp is on the roof and tried to perform repairs or maintenance on these things - I worked summers until age 24 for Dad and it was mind boggling how hard some of this stuff was to work on when a few common sense changes would have simplified things and made the equipment hold up much better. The newer the designs, with a few notable exceptions, the harder for us to work on.
The point to this long spiel is this - I am willing to bet that a boiler built to 1920s design with modern boiler materials would be just fine. Proper operation and maintenance are the key to safety on these things.
Another thing Dad pointed out involves government inspectors. Having had to rip out thousands of dollars worth of correctly done work on an installation just because an inspector said we had to in order to pass inspection and then go to another job site and see the crap that the same inspector passed has made everyone in our clan think they do that crap just because they can. My Dad is honest to a fault and both he and my brother had/have more business than their companies could keep up with - all word of mouth because they did/do good work and are honest. Not all inspectors are bad, but we have run across enough jerks who like to use their power just to do it. So I am willing to be there are good FRA inspectors and some real jerks who like having power.
That was more than two cents. I will put away the soap boxes and be quiet now.