They said the same thing about millenials, then they get older, get jobs, and families, and bills, and bitterness, and many look for people to blame and shift right.
Why would that change? Do you see millennials getting massive wage increases and houses they can buy for 2 pieces of string and some pocket lint? Food prices to drop back to when minimum wage could afford 3-6 big macs?
Why would millennials suddenly start trying to conserve the systems that have working harder than every previous generation while struggling to afford necessities?
Hopefully it doesn't happen, but lots of articles point out that millenials simply lag behind other generations in terms of wealth and income. That could mean that there's entrenched bitterness that will last and keep us left, or it could mean that we see a large millenial shift to the right as soon as we are able to save up enough for housing etc.
I suspect we may stay left, and I can understand the bitter pessimism that we'll just never get to those kind of prosperous economic conditions, but I think that in reality, especially once boomer parents start dying and leaving more inheritance to the millenial generation (and freeing up more housing, healthcare resources, etc), then you might see a sudden millenial wealth increase and corresponding rightwards shift.