To stay alive any business has to adapt. And that's what happens in the brewing industry, just like any other. Brewers come and go. In my life time there are dozens that have gone and changed and been take over, and have taken others over. And the ones that have gone completely have, to some extent in the UK replaced by the small 'craft' brewers who themselves come and go, perhaps getting swallowed up in the process.
So Marstons is a brewery business. Just like the international brewers Coors and Asahi, and the bloke down the road with a 2 barrel set up. But they have to make business decisions. For better or worse Marstons plc has £280M of debt which, with this JV, they have now basically cleared as I understand it. So for them it makes good business sense to enter an arrangement with Carlsberg, although clearing the debt has cost them some control by handing some of it to Carslberg in the JV power split
And as far as the old names like Ringwood and Jennings and Mansfield they were taken over because it made business sense to their boards at the time. If it hadn't been Marstons it is likely it would have been another brewery, or they may have just closed down. Even Marstons (Burton) were themselves taken over by Wolverhampton and Dudley (Banks) a few decades back and the stock market quotation was initially Wolves until they made the business decision to call it Marstons because that had more of a national identity.
So all this is just run of the mill stuff in brewing, its been going on for decades, so get used to it. If you dont like their beer, don't drink it!