You're stretching the facts here. Steam Link isn't dead. It's an app. There used to be a dedicated device for it, but it became redundant now that you can run the software on so many other devices, so people stopped buying it, and eventually there was no point in manufacturing more. That original device is now discontinued, but Steam Link is not.
Steam Controller may or may not be dead. There is some evidence that Valve might be working on a second controller. And even if not, the Steam Input software which directly came from that project is alive and well, and has become a very important part of PC gaming.
As for SteamOS, I'll leave that explanation to someone who knows the topic better than I do.
That leaves Steam Machines (which did fail) and Episode 3 (which is a game and not platform-related, so of course it doesn't make me think my library is in danger).
While the formulation "abandoned" is not true for most of them, "not support them to the fullest potential" probably is.
Steam link had pottential for more (a platform where they have 100% controll of ui/performance), but here you could argue that the potential that was missed was not worth the money (probably, since nobody would like to buy it it its an enthusiast thing)
Steam controller: yeah, no. Here they missed a lot. Maybe the point was always: lets have a controller for pc, done.
But they could have developed games that use the controller to its fullest and would not work as great with others.
Steam os: Krejleee made a great post, still, what they wanted and what they did where 2 things, and they could have pushed the concept way further. Again, feasability (money) is probably a reason, why they decided against it. Probably till they would come to a place where it would do more than it tit (with its potential, a new platform), the chance would be there that streaming would be to prelevant. And wine got better... and stuff.
Essentiallythey saw other developments, and said: probably not worth investing more in it.
I think the thing is, they had a point where they where not shure about the future of pc as a gaming platform, started a view things, and saw other developments, and now they focus on VR.
And im not shure why i wrote this... im not really arguing against your points, and now i dont feel like i've said something relevant. but i will post it anyway =P