I don't think it's completely true. There was an interesting concept discussed in the past : a miner network backbone (direct connection between big mining pools used to propagate blocks, clients connect directly to mining pools).

The concept is interesting and I guess that such a network could work. But it has a major drawback : the network becomes more centralized with less redundancy in the architecture. As a consequence, attacking such a network would become easier. I guess a better solution is a miner backbone cohabiting with full nodes which provide redundancy and a better resilience of the network.

So, as stated by CJYP, even if mining becomes more and more centralized, full nodes remain very important for the security of the network. Their role is different but not less important.