In Bitcoin, the probability of the next TB being an attacker is proportional to the percent of the peers the attacker controls, thus there is never 100% contiguity of control unless attacker controls 100% of the peers.
Bitcoin's 51% works like that: you create your own blocks in secrecy, however many you want, while you have the 51% advantage, and then release your chain. It will be longer than the honest one, so all peers will abandon the honest one and take yours, with whatever you put there. Not only you have continuous control of TBs proportional to your hashing power advantage period duration (which is the cost of the attack), but you can reverse the already accepted transactions, which you can't do in your weak attack on decrits.
I obviously misspoke above (because I was tired). At 51%, Bitcoin is a hard fail and can be forked.
And you are apparently correct that there isn't a hard fail fork vulnerability in Decrits. We were wrong about that, see my prior reply to Etlase2.
Nevertheless I don't know if I like the tradeoff that Proof-of-Share presents, with outages (transaction delays) at much below 51%.