Life Expectancy of Power Supplies
You can't tell the expected life time of any device from its looks.

In your specific case, there's a few things that I'd expect would be the most likely to fail:


defects due to surges on the power source
aging of capacitors
semiconductors due to heat due to dust on the cooler


The first one is totally out of your hands, usually. The question how bad a grid malfunction must be to fry your SMPS depends on the design of your SMPS, and, simple luck. Still, from a pure stochastic point of view, you must expect anything to go wrong once in a while (lightning strikes, car crashing into power line mast) – but that while might be half a century, or half a month, depending, again, on odds totally out of the influence of the SMPS.

Of course, you can limit the effect of things going wrong by sufficient input protection – but that's something that will only be spec'ed for larger, much more expensive supplies than yours.

Then, we have aging and failing probabilities. In an industrial environment, you'll usually find a measure called "MTBF", mean time between failures, that captures exactly that. That number should be higher than what you need it to be, under a heavy-load usage scenario. It would not make that much sense to estimate MTBF under lighter load – a) could you have used a smaller SMPS then, and b) it's simply pretty hard to guess whether things would last longer; I don't really see why a fixed-frequency SMPS would have anything be less stressed in a 1% load vs a 90% load scenario, iff cooling is adequate.

So, if you need that reliability, buy something that comes with an MTBF. That number is pretty hard to honestly and qualifiedly estimate, and only large suppliers with significant experience should be trusted on that. 


  Although we are buying them from a sort of responsible supplier. 


Lol,  OK, so, not that supplier, but a fundamentally responsible one. Yes, there will be a cost factor of > 2.

You usually don't buy such components for consumer electronics; the price/benefit trade-off simply doesn't allow that:


  If yes some other electronic systems like TVs that are on ac power for many years how had solved this problem?


Not at all? Ok, of course, as engineer, you try to build the best SPMS you can with the things given to you. You correctly pick your components with some headroom in their specs so that of the thousands of devices that will be build, the outage probability is low. But if a friggin' 400€ TV fails in the field after a decade … no one cares. That's about the upper end of life expectancy of consumer electronics anyway. If it fails after 5 years, you don't even infer a bad reputation. If it fails after 2 years, well, look that these cases don't accumulate, and if they don't, you're fine as manufacturer; if it fails within warranty, still cheaper to exchange 1 in 1000 TVs than to build 1000 TVs that are 1% more expensive, but fail years later on average. 

Luckily, economics might be on your side here: while any solidly made SMPS will be more expensive than your "sort of responsible" supplier, if you can get access to supplies for mass produced higher-end devices, you'll benefit from the fact that rigorous testing has been done on these, but the mass production distributes the cost of these among more devices, and they still are relatively cheap.

The comments already mention TDK and Murate as typical SMPS OEM, but I'd simply go and open a device that has a similar usage profile as you have – in your case, look for a broken high-end office printers/copiers, and then for the names on the power supplies.