Digimon Tri was marketed as an ambitious return to Digimon's roots, Digimon Adventure, and the beloved characters we knew way back at the beginning. However, we didn't get a celebration. Instead, we got a clusterfuck of epic proportions the writers had to fix with a massive Deus Ex Machina, due to the fact that, otherwise, they'd have outright failed to achieve anything except to further the villain's plans.
Now, I agree the original seven, later eight, children were not paragons of virtue, and more than once, they made mistakes that the antagonists abused, but the sheer amount of failure that happened in Tri made that look like a good run.
All of their flaws from the original series were magnified and amplified, with even Hikari, who was the usual Deus Ex Machina, having her own status as such subverted and corrupted. By halfway, you just knew they weren't going to be successful. By the final film, you were wondering how the hell anything resembling victory could happen.
And it did because... apparently the new member's Digimon ended up with the entire backup file lodged in it's code, reversing, mostly, the cosmic reboot, and it's deletion undid all the screw-ups the main team did. It reminds me of the end of Time Bandits, where God appeared, snapped his fingers and Evil abruptly lost, just because, well, he did, so there.
But we're not talking about the inept cousins of Snow White's Seven Dwarves, we're talking about the original generation Digidestined, who managed to fudge a way to defeat a super-virus twice, once without their full team! But apparently, with age comes stupidity and laziness. To go with an ending where 'The bad guy lost. The End.' is the coda, well, that's not how writing works. And that's why Digimon Tri failed. Since the writers literally went with the good version of 'Rocks Fall. Everyone dies.'
The final movie felt like a Dungeons And Dragons session where the GM went 'While you were bickering, the princess you were trying to save escaped her imprisonment and hit the evil wizard over the head with a heavy pot, then left to go tell her father how inept you were. When you got back to the palace, they refused you entry, because you didn't actually complete your quest.'
Yay, Someone won the day, but it wasn't you. You spent all your time making the job harder, until there was no way you could win without the major antagonist choking on a dark pretzel or something like that, saving you from having to actually challenge them.