I vaguely recall Liverpool coming within a couple of points of winning the thing last year despite preseason odds of 33:1. But I think the point people are going for regarding predictability is that yes, I can predict the Premier League winner this season with roughly the same accuracy as La Liga.
The difference is that I can also predict who is going to win La Liga in five and ten years. Real or Barca have taken something like 25 of the last 30 seasons. Not only has the English top flight generated twice as many winners in the same time period, but literally no teams have managed to achieve the same kind of dominance.
The Spanish system is literally designed to keep the same two clubs at the top of the pyramid - which is what people are talking about when they call the league dull and uncompetitive.
We can revisit this 'debate' after 5/10 years then and see how the situation is then.
There have been 5 different EPL winners in the Premier League era thus far, with ManU far ahead.
In the same time period, there have been 5 different La Liga winners also, with Barca and Real sharing the majority of them.
That is what has happened and thus, what we can go on such info. at this point in time.
However, I will concede that Barca and Real's dominance in both Domestic and Europe is unhealthy, and as you said, can be a real danger to the league going forward if there aren't some changes to TV deals, etc.
That is something most unbiased Spanish footy fans would admit, no doubt. However, that is the future, and thus, to base an argument on something that 'could' happen doesn't address the issue at hand whatsoever...