You are still ignoring everything else that impacts a boiler's life apart from water quality. That is what is dangerous. I'm not looking to offend you but to caution everyone from a laissez-faire approach to safety. We can't develop an attitude where we say boilers can stay in the frames until a major failure. A boiler should be lifted and rebuilt at an appropriate interval as determined by usage and condition to allow proper inspection and repair if necessary. This could be ten or twenty years or any other amount of time but it will fall due for every boiler. That is why such works are stipulated in many boiler codes.A boiler in daily service for 19 years obviously hasn't been through that many heating and cooling cycles.OK Mr Fish, firstly a little background about me, Worked as a fulltime Fitter/Machinist for the last 17 years for 2 Heritage Rail Organizations one mainline, one tourist railway, Volunteered for 3 more in addition to those who I have been employed by.
Again you miss the point. Boiler water treatment is hardly ever going to be perfect. Yet you can have perfect water treatment and a boiler, particularly a firebox, will still deteriorate. Heritage locos naturally go through many heating and cooling cycles from sporadic operation. No amount of careful treatment and washouts preventIs the corrosive effects of fuels and combustion.
I've volunteered at a heritage railway with a boiler water treatment plant more advanced than a modern drinking water facility. They still have their own boiler shop, they still have to rebuild boilers every ten years or so and they will go on doing so as long as they continue operating conventional coal-fired steam.
What you seem to be implying is actually dangerous.
Over those years I have seen the effects of incorrect/ineffective and No Feedwater treatment in both boilers in daily service and infrequent use, and in all cases If you get it right corrosion and scale disappears, this is no pie in the sky stuff it can and has been demonstrated many times in the past.
With regards to the use of sophisticated plants I would argue that this is not necessary as there is an old saying that goes "it's not what your putting in the Boiler that matters, it's what's in it" and if there are still being rebuilt every 10 years said plant has been ineffective.
There used to be some good articles an Feedwater Treatment here https://5at.co.uk/index.php/modern-steam-2/principles-of-modern-steam/porta-s-water-treatment.html bottom of page but no longer working.
There is plenty more info out there for those willing to look for it and learn from it.
In ending I find your comment "What you seem to be implying is actually dangerous." extremely offensive and insulting.NSWGR3827