I 100% agree with you that the value of cryptocurrencies comes from network effects. However, there is also a force that counteracts that namely scaling (or the lack of it). Yes, for every user the value squares ("Metcalfe's law") but also for every user the network value decreases because of network congestion albeit not as much.
On a side note: That's why IOTA is interesting to me because counterintuitively the network becomes faster the more users join (not shilling, just arguing from a technical standpoint).