Delay-gradient congestion control

Especially when combined with traffic from systems that don't use the same algorithm. If systems that use this back off before the congestion is bad, but other systems don't, these systems will not get their fair share of bandwidth (as noted in the article), and if it stops backing off because it detects this situation, is it really solving the problem?

There are a lot of things besides congestion that can alter the RTT of a connection (including server performance at the other end) so it's not clear how reliable this signal will be in the real world.

It's worth testing, and getting it into the kernel as an option makes it far easier for people to test it. But it's far too early to say that it's the solution and will lead to a packet loss-less Internet.