I think in absolute terms it should be easier to become a millionaire, as time goes on, simply because it means less, thanks to inflation. When I got serious about investing for retirement, in 1999, $1M would be like $1.5M today. Conversely, $1M today would be like only saving up around $666K back in 1999. Back in the early 60s, it would have only taken around $125K to be the equivalent of a millionaire today. Or, to have $1M in 1962 would be like having $8M today.
That aside, I think as a whole, it might be harder for millenials, and the generations to follow, to become millionaires, when adjusting for inflation. For one thing, college costs have gone up considerably. I went to the University of Maryland, and I remember my first semester, fall of 1988, was just under $900, plus books. By spring of 1993, it had shot up to around $1500. That was faster than inflation, and we thought that rate was bad back then! But, today, I think it's close to $5,000 per semester, for in-state.
Granted, a lot of people go to school and get useless degrees, or spend too much time goofing off and running up bills. But, even "useful" degrees aren't cheap.
I think a bigger problem, however, might end up being shorter career spans. Many of the younger generation I see today are starting in their careers much later than my generation did. It's taking them longer to get through college, and when they graduate, they have more bills to pay. But, age discrimination is still a major thing. It's more common than we want to admit, and it's hard to prove. So, I can see many of these younger people starting later in life, so they don't have as many years to build up their resources, but they still get cut down in their 50's, when, if things were going their way, they'd otherwise be in their peak earning years.
Also, I don't have any evidence to back this up, but I have a feeling it's more common these days for people to wind up in a job that pays well, but isn't that secure. So they have a good run where they make that money, but then they get laid off, and have to burn through some or all of it, before landing another job. Lather, rinse, repeat, until the whole age discrimination thing sets in.