These type heaters can last a long time and old histories of vaccum tubes failing were mainly due to the technique used to get heat-pipes to be encapsulated inside. The failure point was the copper glass junction involved. This is not featured in any new technology i have seen and tubes are glass to glass now where the envelope is sealed, like lightbulbs and medical vials and use a high temp polymer plug to hold the heat pipe.
It looks like the heater you used as an example has open top evacuated tubes that allow the tank water to circulate (thermosyphon) through them. Some of these use heat pipe tech and can be out in freezing weather.
This type has moderate frost protection, but are not considered in climates that freeze or have long hard winters.
The tank does have 2 inch insulation and a stainless tank, so as far as construction it is awesome and these are used all over the world, very poular across china.
It cannot be exposed to pressurized water. It is more or less a batch heater, but could be auto filled with a float valve, regulator or other rig.
The heater will likely last 30-100 years.....without direct mechanical damage occuring as hail or falling tree limbs, wind blown depbris, etc. It will also greatly outperform all but the most carefully built homebrews of the same collector area and will store water at temp way longer than most homebrews also.
I have dozens of evacuated tube dhw and space heat collectors installed in the field from 2004-2008 and have absolutely no reports of failed tubes that were not struck with something big, fast and hard..... except for some early mixups from the manufacturer where the heatpipes copper tube and bulb assembly were water filled with water instead of acetone as a working fluid. They exploded fantastically requiring major cleanup!
Evacuated tubes can be had feeding a manifold that is a closed loop to storage and exchange, that way you can run pressure and keep your tank inside. Again some of these tank types have heatpipes and can be pressurized.
I am partial to flat plate collectors home made or store bought, they look great, have no vaccum, do not project glass in a 20' radius when broken and so are repairable with solder, sheet glass, foam sheets and screws.....if they were to need it. Flat plates i have re-hab repaired were 30-40 years old and are ready for another 40 years. Aside from insulation breakdown and acid destruction of the waterways, they could last hundreds of years in service if the waterways dont erode. Not sure how many gallons could pass before the pipes would be dissolved, which would be eventual, but at least a hundred years or more based on my inspections and educated guesswork.
A homebrew generally cost a healthy chunk of money for materials and i generally would buy used or new collectors unless heating air. Even a plastic pool heating collector for $200 (4'x20') will outperform every single home baked multi coil black poly tube and saran wrap monster that i have seen.
Sorry guys, by the time you buy hundreds of feet of poly tube and try to tack it up somewhere, then attempt to pump it at a proper rate, i can strap a collector down and have 50 gallons 150 deg. F water steaming by noon, thermosyphon, drainback or batch.
$200 and some used lumber and screws and a chunk of drinking grade hose connected to a pickle drum with bulkhead fittings. It will last a couple decades, but is not freeze proof or a winter performer without a cover.
So home brew balance of system, especially tanks, mounts and heat exchangers is easy, but manufactured collectors are worth every penny in my opinion and based on observation. Copper glass and aluminium simply provide lasting performance and good looks.
Homebrew pressurized batch heaters (water tank in box) are probably the best project wize as far as home brew and do not require specialty techniques, fixtures and tooling, ovens etc., all materials readily available new or scavenged. Not freeze protected without a shutter.
You can buy evacuated tubes with heatpipes or flat blate absorbers as a component and brew everything else, but results vary and used flat plate collectors whole, run $200 and new $800-$1200, worth every penny.