You could provide a way to keep those exotics relevant without completely resetting the progress the way they're doing. Believe it or not, there are multiple ways besides the one they plan on implementing. Besides, and I've said this before, the real reason people are pissed is that it doesn't matter if you upgrade a fully maxed exotic or a brand new one, the result is the same. It would be one thing if these were legendaries, but the time sink it takes to max an exotic is extensive and now they're basically saying "lol oh well". And again, they wait to put this info out a few days after an extensive overhaul of the upgrade system where people have effectively wasted valuable materials upgrading exotics in anticipation of using them with the new content.
I won't say there couldn't be a better implementation, but I just see the logic of their decision. It makes sense, is it the best way? Likely not, but it makes sense more so than making them worthless which is what I previously thought would be the case.
It's also not so bad if you stocked up on the telemetries Xur sells every time he passes by. Though I can see why many likely never bought them.