I'm actually astounded by all the "questions" on this topic. I think it's pretty clear exactly what the intent is and what will happen with the cards:

There's 3 United cards today.

The Explorer is clearly targeted as a direct replacement of the MP Select card. Similar benefits and similar cost (and I can't believe people complaining about $10/yr more). It's also clear that it's nearly an exact duplicate of the Onepass Plus card in terms of benefits. Very real loss in terms of 3x points and the 5k MQMs, but I'm sure they're not coming back.

There will be a replacement for the lounge card. It will have the features of (drum roll) the exact same card on the CO side, just like the Explorer is a copy of the Onepass Plus (with maybe a minor tweak or two). It will probably give you 1k flex MQMs for each $5k you spend, just like the CO PP, which means it's pretty much better in all ways that the UA card (but it costs $20/yr more).

The only real question is if they will be releasing a low-end card to replace the UA Visa Signature $60, but that's probably doubtful; all the other airlines only have 2 cards, including CO. So hold on to that one if you want. Though I don't see a reason to use it when the CO (er UA rather) Explorer is only $35/yr more.

No mysteries here and generally fair trades, unless you're like that guy who buys $21k of UA tickets in a month, who, I will admit, got screwed. For the rest of us it shouldn't matter much.