Have you ever made the BART–Caltrain connection? Nobody in their right mind is going to do that if their final destination is Moscone.

It takes the better part of half a mile to get from platform to platform, and originally I thought the connections weren't timed. Turns out it's worse. BART refuses to do timed connections on the weekends, and on the weekdays they pretend to time it. Connections from Caltrain to BART are 15–20 minutes. Going from BART (e.g. the airport) to Caltrain is not "timed" and is a 20–30 minute transfer. Icing on that shit cake is that BART doesn't run when the first few and last few Caltrain runs are made. So, yeah, nobody will do that. There's no reason to unless BART isn't running, in which case you're reliant on taxis/ubers or one of the scarce SamTrans buses that was gutted to pay for BART to get to the nearest Caltrain station (Millbrae).

If they're coming from the airport they'll take BART to Powell and walk to Moscone or do the ridiculous transfer at Powell to the subway to nowhere and get off at Moscone station.

It's all moot though as BART's not even stopping at Millbrae today because reasons.

I've made that transfer. It sucks but it's still a relevant option, especially for people who live next to a CalTrain station. A CalTrain that goes downtown would likely get them to make it easier for airport passengers to get to Millbrae. Because of that a CalTrain that goes downtown would be worth a lot, but still a tiny fraction of the proposed budget. I hope it doesn't happen if it's going to cost that much.

The conference attendees when at the airport have their luggage. The airport hotel is a more realistic scenario. Or a CalTrain accessible hotel. And some of the airport hotels have good access both to CalTrain and the airport. I think more would if this extension were made.

  people who live next to a CalTrain station

Right. Nobody's taking Caltrain from the airport to a conference at Moscone.

  Or a CalTrain accessible hotel

How many area hotels offer shuttles to SFO? How many offer shuttles to Caltrain? Airport shuttles will stop at the international terminal which is where the BART station is. Shuttles to Millbrae (Caltrain) don't exist ergo nobody's taking Caltrain to Moscone.

Even if there were shuttles to Caltrain, it's a much shorter walk from Powell (or the Moscone station) than 4th and King or the transbay terminal.

You said nobody in your original reply to me and reiterated it in your other two replies. Are you sure that the vast majority of the conference attendees from other states fit the same profile of staying in a hotel downtown? Maybe they are but I would like to see some data.

I said nobody is going to take Caltrain from the airport to Moscone. In fact I looked at three hotels near SFO and they all offer shuttles to SFO but not to Millbrae. Who's talking about downtown? I have no idea what you're on about.

That's because they're right at the airport. Look at the hotels with airport in their name that are within a few miles of the airport. A lot of them are closer to a CalTrain station then the airport.

https://www.hilton.com/en/hotels/sfosoes-embassy-suites-san-...

  Please note that we offer a fee-based shuttle to San Francisco International
  Airport SFO, from 5AM-10AM and 5PM-10PM. Please reserve the shuttle using
  the link below.

That's closer to south city Caltrain than the airport, and yet… nobody is taking Caltrain to Moscone. Judging by that neighborhood nobody's walking to Caltrain either. Caltrain to Moscone isn't a thing.

Yes and hopefully it won't be a thing (just something that happens enough that it's not nobody). But the extension to downtown is attempting to make it a thing. They're hoping that people who take it downtown will take it to go anywhere that they consider downtown accessible. Part of the plan is an underground passage to the Embarcadero BART station. I think the planned budget is ridiculous but I think it would be used at least enough for it to be considered a thing.