I view pay to win as a bit of a sliding scale. Pure pay to win would be where you cannot achieve something by the application of time and/or skill and instead must purchase whatever "it" is. At the other end of the spectrum are games where you can pay for things that don't affect the gameplay experience directly, such as paint jobs and other accoutrements. I do not see pay to win as being a "yes/no" status, it is more complicated than that.
Expansions fall somewhere (not very far imho) along the line because you cannot reasonably expect to compete without maintaining your version of the game. However it is important to note that: One, the price of the expansion is largely the same for each person, sure there's some variation but if you are being reasonable you need to compare people who bought the game at the same time; And Two, the motivations behind charging for an expansion are much more clear cut. That is to say that in order to justify the new expansion, and/or develop the next expansion, you must bring in income as a result of the money spent on that expansion.
Contrasting with a "money = time/effort/skill" approach and you have less clear cut motivations and a potentially significantly different pay scale. If an ED expansion is £20 (I don't actually know what it is at the moment) then you know that in spending that £20 at that point in time that you are in the same position as everyone else who bought at (roughly) the same point in time. If you can buy e.g. credits, ships and other "desirables" ala Star Citizen, then it becomes much more complicated until you reach the absolute upper reaches of spending, which in SCs case is 10s of thousands of dollars. There's a massive difference between being able to afford a £20 expansion and being able to spend $20000~ to ensure reasonable parity between players.
If I have low time and high money I can skip content to maintain parity with my "rivals". If I have high time and low money I can work at it harder to maintain parity. If I have high money and high time then I have the best of both. 2 out of 3 of those scenarios involve spending money. With the expansion approach there are only 2 possibles: I bought the expansion; I didn't buy the expansion. It is much clearer cut. Not only that, but the valuation of time becomes an issue because if e.g. I have 6 hours play time per day, and no money vs I have 1 hour play time per day and a good amount of money, is a possible scenario then there's a weight of responsibility on the developer to determine how much real world cash equals how much in game effort. Tricky.
Ultimately the concerns about p2w boil down to parity...What an individual needs to put in (be it time or money) to stay on par with their peers. As a result I'd argue that SC is much closer to "pure" p2w than ED is. People generally find it less objectionable when someone has put in effort to achieve something rather than just paid for it.
