One definitely should use solid structures, metal or wooden. The damned thing cost ~10$ and I didn’t have time to build a proper support structure at the moment. I meant to use it only as a temporary solution, which I forgot when everything was fine.
The design of the arch itself wasn’t the problem. The interconnecting pipes were only 1-2mm thick, so there was no way it could possibly support the weight of a flourishing grape vine.
It was marketed as a “rose arch”. I guess it could’ve handled this purpose without any problems.