I agree with you that it's usually true, I just don't agree that it has to be true. When a company is well funded and focusing on userbase, that's great. When they need more money, it might not be. I know they are totally different types of companies, but look at social networks - when they are expanding they are good for everyone, once they decide it's time to earn more money from their exist users (focusing on the user) that's when it gets worse.
In Ecosia's case it's different I think - without accounts and nothing keeping you there, every customer today is a customer that needs enticing again tomorrow.
The problem with social networks is that they decide to earn more money from advertisers by selling the users. I don't know how a company can say "I'll only ever want a specific kind of customer", but at least saying "I'll never want to sell" is a step in that direction.
In Ecosia's case it's different I think - without accounts and nothing keeping you there, every customer today is a customer that needs enticing again tomorrow.