The relative value of any particular PoS coin is directly proportional to the size of the network of the coin you are measuring, divided by the size of the networks of all PoS coins. Since the number of PoS coins is not limited, and the network size of each of them is not limited by competing for a scarce resource such as energy, the denominator will grow much more quickly than the numerator, leading value to $0 over the long term.
The relative value of any particular PoW coin is directly proportional to the size of the network of the coin you are measuring, divided by the size of the networks of all PoW coins. Since the network size of each PoW coin is limited by competing for a scarce resource (miners and energy), the size of the networks of all PoW coins is limited. When a new PoW coin is created, it must take from the pool of miners/energy by reducing the size of another coin's network to increase its own. This means the relative value of any particular PoW coin is much more resistant to being drained as new PoW coins pop up. The new coins will have to fight for resources with the existing coins, which is generally very difficult to succeed at versus large coins such as Bitcoin. Thus, well established PoW coins will generally have a stable value regardless of the introduction of new PoW coins.
Of course there is an unbounded number of potential altcoins for both models, PoW and PoS and since PoW is, by design, very work intensive you will soon reach actual limits because resources are bounded.
Nevertheless PoW [Edit] of course i mean PoS [/Edit] also has resource bounds (network transmission, disk space, etc) so the models are actually equivalent, one merely allowing more concurrent coins than the other.
But this has absolutely nothing to do with coin valuation. By the logic of your argument an almost infinite number of small coin networks renders any large coin network of 0 value (say a 1 million user network in relation to 10^30 networks of coins with 10 users each also effectively leads to a value of practically 0 for the 1 million user network)