They never officially discontinued the mini line in the first place. They just didn't release new models after the 13. The announcement that they discontinued it was that they were discontinuing production of the 13 mini, which they had kept making alongside the regular 13 after the 14 lineup was released. Up until the release of the 15s, you could still buy the 13 mini new from Apple. Discontinuing the 13 mini killed off any way to buy one new.

There's still strong rumors that they might use the mini frame for the next SE model. It has pretty close to the same external dimensions as the 3rd gen SE, but would bring more screen real estate in the same package since it would be edge to edge instead of have the bezels. And it would also fit the rumored "release a mini every few years" strategy since they only release an SE every couple of years anyway.

The SE cannibalized mini sales because, for the most part, people that wanted the smaller size cared more about price than features, and the SE was the same size and quite a bit cheaper. Replacing the old iPhone 8 based SE with an iPhone mini based SE would kill two birds with one stone. It would let them use up a bunch of the excess stock they have laying around due to the poor sales, would bring the mini to a lower price point which would make it more popular, and would remove the competition between the SE and the mini.