Indeed, mathematically Barcelona still has that chance, but if no team succeeds in stopping Real Madrid until the end of the season then everything will be in vain. Apart from that, Real Madrid doesn't just need one defeat but at least four defeats if Barcelona wants to overtake them at the top of the standings. With that, it would be very impossible for Barcelona, but I have little faith that sometimes miracles do happen.
Well, maybe it's true that the only title they can win this season is the Champions League, but they again encounter problems when other strong teams are in this round and things will be difficult for them again. If they manage to beat PSG in the next champions league they will face a much stronger team and with their readiness completely out of control at the moment, I doubt they can go any further.
Barcelona opportunity is still open, but waiting for Real Madrid to lose points in the next four matches is impossible. After losing to Atletico Madrid in the first half of the season so far, no team has been able to beat Real Madrid, Barcelona must do it themselves in the upcoming El Clasico match to reduce the point difference. Finishing in second place in the La Liga standings, and fighting to reach the semi-finals by beating PSG is Barcelona main target now, while Barcelona chances of winning the Champions League trophy also seem very low.
If for example, Real Madrid gets a defeat like that then, do you believe at the same time Barcelona is able to maintain consistency with good results in each match, I personally doubt it. So yes, it can be assumed as a miracle if for example Barcelona managed to overtake Real Madrid because afterall, I am sure even though their pointsgap is still fairly possible to be closer. But yes, the reality will still be difficult to overtake Real Madrid at the top of the standings, because ofcourse Real Madrid is also still very capable of maintaining its top position until the season ends. At least, it doesn't matter if Barcelona fail to get the trophy this season, after all, Barcelona will also still get a Champion League ticket next season and that way, then Barcelona can improve its quality next season.