There's probably enough demand to feed the top foundries, but competition makes a difference. Firms that are candidates to be an Intel customer will want to compare Intel's deal to the rest. At minimum Intel will learn a great detail about the potential client's products yielding good competitive intelligence and at best they can cherry pick some good customers. Of course some of TSMC's customers will use Intel to hammer TSMC on price so Intel's foundry business brings new pressure that didn't exist before.
There's enough growth in the demand for foundry chip fabrication to account for Intel's new customers while still giving TSMC, GloFo and Samsung more demand than they can handle. Intel taking on a few small foundry customers makes no difference at all.