They don't offer trading, they offer swap.
They're the same thing. Swapping, exchanging, trading assets, they're all synonyms. What you're referring to is how regular one trades, which in the case with retail trading is probably everyday?
Therefore, if these decentralized services cannot offer a full-fledged alternative to centralized pairs, one cannot say that one should forget about centralized services and switch to decentralized ones, since the use of centralized services does not always come down to a banal purchase, sale or one-time swap of one asset for another.
Well, sure, truly decentralized exchange doesn't come with all the commercialized shitcoins a centralized exchange provides, but I don't believe that someone who wants to buy bitcoin cares much about that.
People come to centralized exchanges to speculate, not to exchange
Not necessarily. I know lots who simply wanted to hold some coins for the long term, and they registered on some centralized exchange to accomplish it, and you know probably too. Decentralized exchange is for those.