Goldman sachs and govcoin can never replicate some of bitcoins best attributes:soverign, immutable, no KYC. and extremely unlikely to replicate other features that make bitcoin so great : limited , disinflationary.
Once you know this , you will have no fear of these private blockchains.
Who says it has to even be private? All they have to do is clone bitcoin and premine the shit out of it, market it as the drug and terrorism-free version (loss of anonymity is a feature, not a bug), and prop up the price for a few months. If they hire K Street lobbyists to encourage USG to treat their coin a little more charitably and our coin with a little more hostility, it's off to the races.
We'll still be squabbling over SegWit and 4MB when their nodes are the size of Google data centers processing millions of transactions an hour. They'll be like a hot IPO and we'll be de-listed and relegated to the penny stock pink sheets. And we'll deserve it.
Again BJA simplifying a possible scenario to the degree of absurdity....
Lot's of steps need to occur before this particular BJA scenario were to even be possible of playing out, and sure, if there is a competing Alt coin that seems to be more profitable, many of us BTC advocates, including yours truly, would likely be jumping onto the more profitable one (at least diversifying our investments onto the apparently more profitable alt).
Nonetheless, your pie in the sky speculation is nearly pure absurdity, because it probably has less than a 1% chance of playing out as you describe. Yet, the scenario may resonate with a few readers merely because it is possible, but even if something is possible does not make it very likely or something that we should be acting upon at the current moment... because, at the current moment, it's ridiculous.