Wikipedia's costs do not grow linearly with the amount of people accessing it. Signal has to run servers and store the data for every user it has.
Also, if the wikimedia foundation closed tomorrow, Wikipedia's content would live on mirrors and IPFS gateways. It would still be useful - perhaps even more efficient than it is. Signal's outage yesterday that this logic does not apply for them. If they run out of money or infra, everyone is out in the cold.
> This is the best arrangement for users.
I'd argue that the best arrangement would be to stop hiding the operational cost per user and charge them for it. Donations can come later to cover users who can't afford the service. WhatsApp was doing just that with their $1/year fee and subsidizing market growth markets before Facebook got them.
I'd also make the case that part of the reason that we (as users) should not accept a centralized solution is due to the fact that a distributed/federated system gives room for competition and diversity. Switching from WhatsApp to Telegram or Signal or iMessage brings us no real progress in the aspect that we are still at the mercy of one single monopolistic entity.