In this case, you can't have the left if one of your opponents called a next loner. You would have already turned it down.
True, and I caught that this morning myself. However, I did also ask if you would make the same call if you were already holding the jack and the ace was the turn card. That was probably already in my head when I was typing up that statement. Nobody has answered this yet.
I agree that it makes sense to pick up when playing against expert opponents. You have a guaranteed loner, which means that it is likely that someone else has a very strong hand in another suit, and it's more likely that an opponent will call it rather than have everyone pass and give you the opportunity.
There's no real concern that an opponent will call a black suit. If one of them has a relatively decent hand in black (say four clubs or something like that) and calls it, I've got a decent shot at euchring them depending on what my partner holds and what their fifth card is.
Nobody is going to be crazy enough to call spades unless they're really drunk, in which case I have a surprise for them. (I did have someone try a hearts loner against me when I was holding a Dutchman and two off aces though, which predictably caused their partner to go through the roof and mine to fall out of his chair with laughter).
As you correctly point out, the real risk is that someone will be holding something like the right+2 diamonds and try a loner there. Realistically, the odds that somebody else has a killer lone hand aren't that great. I'm actually holding both black jacks, one of the red ones, and two aces, one of which is red. The odds that somebody is holding a hand good enough to sweep me are pretty astronomical (1 in 1500 or so). Most likely one of my aces will stop the sweep or my partner will stop it.
It's more likely that somebody will call something and take their partner along. Most likely that will be diamonds or clubs, with diamonds being somewhat more likely. I've got a decent shot at euchring a clubs call. A diamonds one will probably make a point unless my partner has the other jack or the ace.
My hand is pretty unlikely to make more than 1 point. Yes, I've got the 2 aces but the ace of spades is unlikely to take a trick unless I'm really lucky on anything but a spades call. This is because 3 spades are in my hand, so there's a max of 3 more spread among my opponents, partner, and kitty. It's pretty likely that somebody has a void.
So the real choice here is between picking up and (probably) making a point or passing and giving the opponents a decent possibility of making a point... with the added gamble that I might be able to make 4 points with a pass. Against an expert opponent, you absolutely want to pick up because they'll never pass in the second round if they don't have reverse next blocked and there is exactly zero chance that anyone has both reverse next suits blocked (although they might have clubs blocked and figure that's good enough at a score at 1-0). Against an amateur though, it is a tougher call.
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