[Osiris]

The only reason proof of work is working for Bitcoin is because of the amount of hashing power put into it.

Nearly every other bitcoin fork can suffer a 50% attack because the network is sufficiently small that one can rent enough hashing power to attack it (see Bitcoin Gold).

Alternate coins absolutely need some other mechanism to secure their chain that doesn't require a critical mass.

[BTCOG]

This is precisely why those other chains are only imitators with no true value. The longest chain is bitcoin, and all the forks thereof are imitations that hold no value or network effect. This will always be the case when someone tries to fork Bitcoin rather than actually do some new cryptographic work.