Perhaps my latetst poll needs some more explanation (apart from what we did in this post).
First of all, the question (how many infrastructures do we need?) relates to access networks, be it fixed or mobile.
Next, it requires a sort of high-level view of competition. OPTA says "two is not enough", referring to a telco/cableco duopoly. So then, how many networks will suffice? Or will just one do (and share it, to avoid capital destruction).
To round off: to support the case for four: Swisscom decided to roll out FTTH with four strands of fiber to each home (see slide 3 in this presentation), for easier competitor access (now of course things get complicated: is this one network, or four?).
Pick your gut feeling!