I agree that Borg was as great as anyone, and this is borne out by stuff like Elo - his 2622 peak Elo is second only to Novak's 2629, and one of only two players to surpass 2600 (I'm pretty sure Laver surpassed it in the 1960s, but we don't have the records).
For me the big question within the larger What If story of Borg is whether or not he would have adjusted to McEnroe, who beat him in their last three pro appearances, including two Slams. But I think we can speculate: they were close matches, and in their case I think the invitationals actually have meaning: In 1982-83, McEnroe won 3 of 5, but Borg won their last appearance in 1983. Meaning, Borg still held his own against Mac, and presumably would have held his own on the regular tour.
I do think that like Mac, Borg would have faltered against Lendl and a younger upcoming field in 1985 and beyond, or at least dropped significantly, so what we're mostly missing is 1982-84...just three years. So we can pad his record with hypothetical wins, but not by too much.
Furthermore, Borg was weak on hard courts. So if we're looking at padding his record with Slams, we probably should assume that he would never have won the US Open. But I think he still would have been the favorite at Roland Garros and Wimbledon for those three years, maybe winning 2-4 of the 6...and then we can add an Australian Open for gits and shiggles...so I'm thinking 3-4 Slams added to his total, so 14-15. Just a guess, though.
Or to be more specific, at Roland Garros, I think he would have fought off a 17-year old Wilander in 1982, and prevented Noah from winning his only Slam in 1983. But I think a surging Lendl in 1984 would have taken it, and he would have been done at RG by then. At Wimbledon, I'd give him one of the three in 1982-84. I don't think he'd have gotten past Mac or Connors at the US Open. If he wanted an AO title, he could have had one in the early 80s, but that's true of Mac and Connors, too. So again, 3 at RG/Wimbledon, and maybe 1 AO if he decided on it.
Borg had Connors number before he retired, and he dispatched him in straights at Flushing Meadows in 1981. Borg wasn’t a bad player on hard courts, he just faced huge performances by lefties who he’d beaten at Wimbledon, between 1978-1980, and of course McEnroe in 1981, who’d beaten him at Wimbledon.
The what-ifs with Borg get complicated. First, I think had he retained his drive and hunger he’d be favourite and possibly winner in Paris from 82-84, and possibly even to 85 or 86, though it’s harder to say. Bear in mind that Lendl was slightly fortunate to get the FO title in 1984, with McEnroe imploding. It’s not far fetched to say Borg wouldn’t have lost that match.
But who knows how he’d react to another Swede joining him at the top? He had two come along in quick succession, with Mats and Stefan. That can affect a player - Boris Becker coughed his own arse up when Michael Stich reached the Wimbledon final in 1991.
McEnroe certainly missed Borg in 1982 - to the advantage of Jimmy Connors. So McEnroe might have reached his 1984 level sooner, and not dipped at all, as long as his great rival was playing. McEnroe revelled in that rivalry in much the same way Rafa did in the early Fedal years.
Like I said, if the measure of greatness was the number of slams you win, then I think Borg would visited Australia every year - but so would Connors and McEnroe. Being on grass might not be an issue for Bjorn - Australian grass played differently to Wimbledon. Wilander beat McEnroe at the Australian Open in 1983, but he never got beyond the quarters at Wimbledon.
It gets complicated but I’d see Bjorn getting a couple or few Australian Opens, 3 more French, and look at how Lendl dominated McEnroe even before Bjorn retired - three straight victories in 1981 alone, all in straights, followed by 4 more wins in 1982, dropping only one set.
However, Borg was similarly ascendant against Lendl. Lendl beat McEnroe in the 1982 US Open semi. You can see where I’m going with that - a route to a Borg NY victory. Not so far fetched, in 1981 Vitas Gerulaitus squeaked out a victory over Ivan in the fourth round, only to fall to McEnroe in the semi.
They’re interesting topics, these alt-history ones. Bjorn left too early, that’s the problem. He was so great we still want to see him win - in our dreams!