I don't think the existence of servers makes it centralized if you can use more than one at a time and run your own 🤷♂️
Discussion
It makes it federated (at best), which is a problem solved in the 90s for IM
The comparison section of their white paper is probably worth a skim:
https://github.com/simplex-chat/simplexmq/blob/stable/protocol/overview-tjr.md
Not seeing a reason to switch to it
That's fair! Neither I have to be completely honest
Reason the idea behind is quite good. The fact is its not realy decentralized. Servers normally used all belong to Simplex. User tun servers are not searchable and yet not forming kind of a decentralized network but maybe thats something yet to come
Nostr uses servers