It's true that, until the bill passes as presently worded, there is nothing to stop a club joining a Super League but you can't seriously be thinking Masters has ruined the reputation of the PL so much (yet) that fans would now want an ESL? Liverpool and United fans, maybe. But there are one or two fans in the country who support other clubs who may have a growing distaste for Masters' PL but won't prefer an ESL.
Also, it would be a hell of a risk. There is nothing to stop the bill being amended to cover the period since it's announcement that would prevent clubs who left before the bill was passed from playing any games in England.
ESL is never going to include English clubs now, imho.