The ads Discord implements aren’t like the ads that track your every move online. This is basic product placement. I’m not even opposed to that as long as they keep it minimal and remove that shit from the paid subscriptions.
Signal doesn’t have anything close to the Discord experience. It lacks basic interaction patterns like channels. Voice calls work completely differently from a conversation standpoint. I don’t think Signal even implements threads, if it does I’ve never been able to find them. The UI for managing things like emotes/sticker packs is a lot better, especially for communities.
The there are the advanced features. Webhooks are used quite often for automation. Forums and discussion boards are used by larger communities. Roles and access control don’t exist in Signal because it doesn’t have a concept of communities.
Discord is much closer to Slack and Teams and Mattermost than it is to Signal and Telegram and WhatsApp and iMessage. On the protocol level, all of that could be implemented by exchanging special messages between devices, but the only app Signal devs want you to use contain none of that. Signal does have stuff Discord doesn’t have, such as encryption and cruptocurrency built in, but neither seem to be dealbreakers for everyone else.
You don’t need to use Discord, nor do you need to use Signal. I have Signal, but nobody else does. You don’t need to convince me, convince everyone else, because my Signal conversation list has been empty for months since I last reinstalled my custom ROM.
I’m sure Signal can be used in the same way as Discord, in the way you can technically run a copy of Microsoft Word on the PlayStation if you stack enough hacks on top of each other. For the vast majority of the user base, that’s not going to happen.