How do journals serve the TCS community?
The process of journal reviewing shakes out bugs. Conference reviewers tend not to look at papers with a fine-tooth comb; the program committee process gives them too many papers to review in too short a time to do that. The main things that one should get out of a conference review are whether there are obvious flaws (these are rare) and whether the results are interesting. A journal review should evaluate those things too, of course, but it also needs to check the details of the proofs to make sure that they all work and are clearly and correctly stated.

If you are treating your journal reviewing tasks as if they were conference reviews, and only giving them a high-level interestingness check, you are doing it wrong, and depriving journal authors of a valuable service. If you are treating your conference reviews like journal reviews, spending the time to go over them carefully, and making sure that papers don't get accepted until all the details are correct, congratulations and thank you for doing such a good job, but I don't think that's the case for most STOC/FOCS/SODA program committee members.

Secondarily, many conferences still have artificially small page limits, and the journal provides space to fill in the details. The preprint version does that, too, but doesn't provide any assurance that someone has checked it carefully.

If I see a paper published five or more years ago in a conference, making claims that are not backed up by full and clearly-written detail in the proceedings, and with no journal version, I am likely to consider it untrustworthy. Is that the future you want for your research?