Although Jones is slowly fading and just had a catastrophic injury he is still a nightmare matchup for Alex.
Jon is not reckless in his standup so it would be much harder for Alex to catch him even when Jon closes the distance and goes for takedowns which will eventually lead to a successful submission be it in round 1 or round 5. Over a long enough time frame Alex will get finished by Jon.
The only question is can Alex present credible threats while the fight is standing and eventually hurt Jon, but that is a big IF with the takedown threat.
Remember Alex didn't have to worry about takedowns that much in any of his fights up to Jon, and we've seen grapplers outstrike strikers countless times due to this.
But the fight probably won't happen, why would Jon risk it now? This is more of a fantasy matchup then a fight that is likely to happen, better questions are how Alex fares against Aspinall, Volkov, Gane and other heavyweights.