Middle class jobs as you call them, aren't disappearing, they are changing.
Take for example, I'm an Automation Engineer. What I program and code for the PLC(programmable logic controller) has to be tested within the system it is designed for, it has to be built(wiring components together) and there is always going to be maintenance on automated equipment.
There are also a lot of things that robots and automated equipment can't do that a human can. Humans will always be needed in an automated world.
On it's deepest levels at least in my opinion, universal income is pretty much one of the dumbest things in the world. If I am getting paid the same amount of money to design, program and implement an automated system as the guy doing documentation on how the system operates, I am going to feel underappreciated in what I do and underpaid for that matter. I have been in both shoes, writing documentation and designing an automated system from start to finish. The former really isn't all that difficult if you are college educated and have some writing classes under your belt. The latter however, that is something on a different level in it's entirety.
Also, look at it this way. Take two of the same exact job and you have 2 people working. One is a motivated individual who strives to really make the world a better place while the second person is a lack luster individual that does just enough to get by. With universal income in this situation, you're completely screwing over person number 1 who is busting their ass.