They’ve added new features, cosmetics, characters and more since launch and I imagine they’ll continue working on it if they’re able to make it sustainable, so they absolutely are providing a service.
You can argue that letting you watch replays doesn’t have any ongoing costs for them, but it probably does: your replays aren’t just sitting on your hdd – the replays are on their servers and your replays serve as content for RankedTV and FriendTV. Maintaining those systems and paying for the storage and bandwidth to host them does cost money. Letting you store and play replays locally, outside the existing replay system, would cost them money to implement as an extra feature. I think the replay system they went with is really cool – above and beyond what most games do.
But for the sake of argument let’s assume they were just storing replays locally and putting a paygate in front of letting you watch them. Even then, I don’t think it necessarily makes sense to argue that it’s only ok to charge for features that have no ongoing costs. What content make sense to charge for and what content cost money to maintain often don’t line up cleanly. They release new characters for free even though they take a lot of money to develop – they have to make up for that by charging for other things.
I also want to say that, as a consumer, it does make sense to try and get the most for your money – and if we’re talking AAA games I agree 100%. But when we’re dealing with indie games, which more often than not struggle to make enough to sustain development, I think it’s worth thinking about it a little differently. Your money isn’t just getting you in-game content, it’s increasing the chance that the game you enjoy will still be here tomorrow.