At its current 20GB and linear growth it is manageable assuming normal use. My concern is that people will find other uses that bloat the chain, or that an attacker will spam it.
It is cheap to embed data at 0.01 mBTC per kB. That's half a cent per kB. 1 MB of data costs $5 and one GB costs $5000.
An attacker can add 10 GB for $50,000 only... or 100 GB for half a million. This is much cheaper and convenient than acquiring 51% of all ASICs.
Regular users are also a threat. Once they discover that they can use bitcoin to store data, you'll have all sorts of love poems etc forever on the blockchain.
The worst potential use is by terrorists and pedophiles. Imagining them embedding videos. No thanks!
