Nebula Control has no free-to-play tier. Every player on a live server pays the same subscription. The why breaks down into three reasons, and none of them are complicated.
The first is abuse prevention. Browser-based games have a well-known multi-account problem. Creating a free account costs nothing, which means creating ten of them costs nothing. Players will do it to grief others, spy on enemies, or even field an entire fake guild to compete against real ones. This can lead to real players quitting, which makes servers smaller and worse. Having a subscription doesn't eliminate the abuse, but it does raise the cost of doing it, which reduces the number of multi-accounts.
The second reason is simpler. The game needs to pay for itself. There are server and service costs that need to be paid, and I have no external funding. My estimates say that at launch, roughly 80 paying players cover the cost. That's an achievable number.
The third reason is that I'm opposed to pay-to-win mechanics. Mixing free-to-play accounts with paid accounts effectively means the paid players are paying to win. A subscription that everyone has to pay keeps all players on equal footing.
Charging upfront limits my audience, so here's what I've done about that. The training server is free. It's designed to give potential players a feel for the game before they commit to paying anything.
I am also building a referral system where a current player can give a one-week trial on a live server to someone they want to recruit. This helps the community recruit new players, and recruits brought in this way are likely to stick around longer than someone who signed up cold.
I'd rather run a smaller game that respects its players than chase numbers and watch the design fall apart.