If you look at the history of hardware as a whole, generally speaking you get one or two dominant leaders, and then you get 5 or 6 people who can follow semi-competitively. The leaders often can hold a 2-4 year lead over everyone else.
In the case of something like sha256, the algorithm itself is simple enough that it’s difficult to really get a lead. So the lead tends to not be as strong and not last as long. That said, the S9 held essentially complete market dominance from 2016 to 2018, so that in fact was 2 years of headway. The team that made the S9 has now made the M10, and that machine is similarly very far ahead of what everyone else has been able to do.
For something like ethash or progpow, the economies of scale are a lot more favorable to larger manufacturers, because they incorporate so much memory. You now have more things to optimize: the silicon chip itself, the memory interface, the memory chips, and the general communication structure and algorithmic strategy. Early on this will make manufacturing a lot more chaotic, as one team developing an advantage in X may be sufficient to put them far ahead of everyone else. But in the long term, to be a competitor you will need to have master tier skill and scale in all major resource categories, which takes more money and a larger team, and altogether makes it much harder for a startup to compete.
I think it’s reasonable to assume for ethash and to an even more significant extent progpow the lead that the industry leader has over the rest of the network will be larger than what we see in bitcoin, and the number of years ahead of everyone else that they will be will also be larger than in Bitcoin, though it will probably not be as bad as it is for GPUs or CPUs.