You basically said it but just to maybe try and be a little more specific here, it is possible to be objective when you say one expansion is better than the other.
Like you said, if you are talking about sub counts or popularity or overall success in terms of the amount of people play it you can be.
The issue is you specifically need to define ‘better’. Because you can think ‘better’ just means more fun for you specifically. You can think ‘better’ means more developed graphics. ‘Better’ can mean easier. If you define what you mean by better (which in the case of the guy you are referring to it was defined to the meathead poster), then you 100% can objectively say it’s better.