I imagine that they'd eventually run out of land to put them on, but from what I've seen, utility-scale solar often sits in places with a fair bit of room for expansion.
The other factor here is that many agreements between a solar power and a utility that buys the power only have 20 year terms. Generally, there's a strong incentive to renegotiate at the end of the term, but frankly, that renegotiation is kind of a mess in practice. It depends on the policies of the state, the utility's interests, the ISO market, etc. as to how that ends up working. Every solar farm is working in at least five overlapping regulatory environments - local, state, utility/retailer domain, the ISO or regional grid, and the federal regulatory environment. Decentralization is nice in theory, but definitely makes it difficult to scale the widespread change that's required right now.