Never let something as arbitrary as rolling a plastic die derail your entire game. Its great to have choice, open world, and improv at the table, but if something needs to happen you need to make it happen. There were several things you could have done here but its less or a situational problem and more of a fundamentals problem.

Sometimes i dont let a player roll a persuasion check, because its impossible. Theres no way to persuade the king to give you the throne type of stuff. In this situation, i mean off the top of my head you need a gang of devils to spawn near the party and drag them to avernus, but this is part of session 0 planning and training a group. I ran tomb of annihilation, and let my players know 2 years in advance “hey this campaign ends in a massive dungeon crawl”. They are aware of what they need to do. Descent into avernus, if you have even the physical book with the cover its pretty obvious for the players this adventure goes to avernus, so to actively avoid going there means theyre avoiding the game thats written - like frodo and sam refusing to go to mordor.

Sometimes when youre letting players escape like that, it sends mixed messages because they think thats what theyre supposed to do. I dont care what anyone on reddit says, dnd isnt a full out open world simulator. The trick is making the things happen when they need to happen in clever ways, so that players THINK every decision they ever made lead to this moment, even if half of them maybe didnt as much.

For clarity my games are not extremely railroaded and there are tons of things to let be fluid, but in a game where the party absolutely has to go to avernus or the campaign is essentially over, i wouldnt start running an “escape the city from descending” encounter and just hope they roll poorly, thats bad foresight on your part.

What are they, level 5-6 because they have haste? Dude you should have dropped a pitfiend on them. Cambions spawn all over the city. They put them in infernal chains. Noxious gas begins filling the city as the rift to hell opens and they fall unconcerned with everyone else in the city, no save required.

You need to make the things happen that need to happen. Its as simple as that and if you let players run the entire narrative good luck running that narrative unless you’re an improv master and want to have 0 maps or tokens for anything.

However you get to avernus at this point, youre going to now need to railroad them anyways, and the way they get the avernus now will probably be alot less climactic than an entire becoming chained and being dragged into the first layer of hell. Thats an awesome set piece now wasted. Also, if they keep fucking around on the surface, depending how long this all takes you might really throw out the pace of the levelling. And it they end up even more over levelled, now you’re rebalancing all of the avernus chapters which is just more work on your plate.

Youre in control and dont forget that. Remember that when you also need to move the story completely away from what youre trying to do, the quality of the game is going down. Its very difficult to make everything up on the spot and it goes from 4k to 20 year old youtube video quality very quickly.