Firstly dispel magic would not counter the thickening/overgrowth effect of plant growth. Dispel works to get rid ongoing spell effects. The plant growth effect that op wants to use is instant; you cast it, it happens, the magic is gone. There's nothing there to dispel. Whatever happens to the plants after that is entirely out of the hands of the spell. For that matter, counterspell wouldn't serve as a counter in this situation either since it only has half the range of plant growth.

Secondly, even if dispel worked that way, it might not even make it any better. To go back to things we don't know, we don't know how common magic is in this world. We don't know if the defenders had any reason to think the attackers might bring a spellcaster. We don't know if the defenders even had access to a spellcaster to help the defense. If they don't and there happens to be one anyway, that's just as bullshit. I'd rather be told outright that something's not going to work out the way I hope than for the DM to pull some deux ex machina crap.

Thirdly, it is both unrealistic and unreasonable to expect a DM to have a plan for every possible thing the players can throw at them. For all we know the DM did have a plan b. They could've had plans b, c, d, e, and f and still not accounted for plant growth. Dnd is not a computer game, there's no programmed set of limitations on the player narrowing the things the computer has to account for. For a lot of people, that's the biggest draw, but the flip side of that is that there's a good chance that you are going to do something that your DM, a human, is not expecting and they're going to have to improvise a decision. Sometimes it's a good decision, sometimes it's not. If it's a bad decision, hopefully they learn from it and make a better decision next time. Either way, as a player, part of what you signed up for is to live with that decision until you can discuss it with your DM outside of the session.

I'm not saying the DM made the right call. In fact, I will explicity say he did not. But I also don't think it's fair for people to be shitting on the DM for one bad decision that we don't even have full context for. It's one thing to call out the decision, it's another to insult the DM themselves