2005 Prius - Right taillight LED going dimmer and dimmer...

EBay is your friend for parts

 

The bumper does not need to come off. It's simply removing some nuts off bolts. Possibly 3 of them

 

Don't blame Toyota! Actually car manufacturers aren't allowed to install changeable LED lights (except interior lights). So they have to use complete assemblies - at least in several countries. But they won't use other assemblies in a few countries, even if it would be allowed, if they don't have to.
I think it's because many people wouldn't choose the right led for replacement. A bulb has information (voltage, electric power input) at the socket, no problem. But for LEDs you need a proper data sheet with voltage, light power output, beam angular, wave length or color temperature (maybe I'm not using the correct English terms) to be sure having the right led.

 

Recommend you do it yourself, buying an assembly off ebay or elsewhere for low cost. Very easy to do with a basic set of wrenches. See this video.

It's good to learn to do simple car repairs like this one, as doing this kind of work will save you thousands over your life. Also very gratifying to fix things yourself Otherwise the dealer's price is reasonable IMO, but I'd never pay them $250 to do something I could do myself in an hour for less than $75.

As far as the other side goes, the failure rate on electronics is generally close to flat after the initial break in period. This means that a 5 year old LED is only a little less likely to fail than a 10 year old one over the same time period. If you've replaced the first side yourself, then if the second one fails, you'll be able to do the job in half the time.

The reason that the LEDs are not replaceable is that they are never expected to fail. LED bulb life is typically around ~50,000 hours. If they were on continuously for this time (which your brake lights won't be), and you averaged 30mph in the car (which is about average) that works out to an expected life of 1.5 million miles. However, sometimes you get unlucky, or water gets into the assembly and corrodes things, so very rarely they need to be replaced. You probably just got unlucky.

 

While LEDs can dim over time, it's somewhat unlikely that the entire array of them would fail at the same time. I'd check for a faulty connection (e.g., loose, corrosion, etc) between the car wiring and the taillight assembly before replacing anything.

 

My 2004 is doing exactly the same thing, the right led's are dimming, one or two are dead and another flickers. Not going to replace it until it totally dies, and I'm at risk of getting a ticket.