Doping isn't the only explanation, but it is the best explanation. It is an argument based on what is likely or probable, not what is certain. Certainty isn't provable without a confirmed doping violation. We are unlikely to get that.
Even when you talk about likelihoods or probabilities, you need some sort of measurements or at least reasonable estimates.
What you have is pure subjectivity, where the likelihood can range from 0 to 1 depending on your gut feelings.
It is subjective to think Nadal has done something requiring explanation in the first place.
It is subjective to think that doping is a good explanation at all, let alone a better one.
A doping violation will tell us he doped, but that still does not address the two points above.
All you really have are suspicions based on some subjective notion about what clean performances should look like.
The opposite of subjectivity - an objectivity, if you like - does not require a specific measure, like numbers. Indeed, you personally wouldn't accept that kind of measure for running either, as you have never conceded that any given time for an event suggests doping let alone proves it. Nor can tennis achievement be measured in the same way as running - by a stopwatch. It is a much more complex sport.
If we look at what we consider expert opinion - by a doctor, lawyer, engineer or scientist, say - we see that it is subjective in the sense it is an opinion but it has credence based on the professionalism, skills, experience of the expert and the criteria they use. It is a matter of considered judgment, which is what we accept from experts in many walks of life. It has more weight than a lay opinion or that of the person in the street. So it may be with an assessment of whether a given level of sporting performance suggests doping.
That expert judgment I have relied upon to form a view of Nadal's doping has come from professional tennis players, coaches, physics, and antidoping experts. For example, we have expert opinion from the former head of WADA, Richard Pound, who said that watching Wimbledon confirmed for him that tennis has a doping problem. (He didn't mention anyone by name, but was implicitly referring to those year's finalists - Nadal and Djokovic). I have referred previously to an acknowledged antidoping expert who says he consider Nadal remains "under suspicion". I also have it directly from a very senior WADA official (a personal friend) that their view is that Nadal dopes and is likely using EPO.
If you want a numerical reference point if not an actual measure for likely doping by Nadal it is in the scoreline. His confining Djokovic to a mere 2 games in the first 2 sets is a ridiculous scoreline, considering who Djokovic is and his form in the tournament. It is similar to a winning margin in an Olympic 100m that would defy credulity - such as 10m. Add to that Nadal's age of 34. Can we see Usain Bolt being better in his mid-thirties than in his mid-twenties? Nadal played an almost perfect match. It requires extraordinary physical skills to do that on clay against an opponent like Djokovic, because, as I have pointed out previously, he has never been able to demonstrate that kind of dominance on faster surfaces, and indeed has something like 11 straight losses to Djokovic on hardcourt. Nor has he come close to doing that to (an uninjured) Djokovic on clay.
I could go on, with other features of the game that evoked scepticism, but the point I make is that an estimation of doping in a sport like tennis cannot be made according to strict and unarguable criteria (although that is difficult to say for any sport); it is a matter of judgment based on an understanding of how that sport is played and the level of the athlete concerned. In that sense, while it remains a matter of opinion, as a judgment it isn't "purely subjective". Nor is my view of Nadal likely doping.