Exploratory Committees are total BS - Ted Cruz did not get a exploratory committee together and got 2nd place in the GOP primary over all the other exploratory committee havers. Trump did an exploratory committee, but in the months between that and his announcement he was still only at 1-2% and was summarily laughed off the stage as a potential candidate. It was only after his real announcement did Trump surge to 1st place and never really lost it (except for maybe a few days to current HUD Secretary Ben Carson).
Most people who run for President basically know they are going to run.
Exploratory committees give candidates plausible deniability. They can start raising money and do as many candidate-like things as they like, will still having an excuse for not being ready to do everything. E.g., not yet having well thought out positions on every issue, or not being ready to debate.
That said, because of how campaign finance law has evolved, they’re being used less often these days, as candidates now mostly use leadership PACs as pseudo-exploratory committees. And most of the Senate (including every Democratic Senator considering a 2020 presidential run) already has a leadership PAC. (Heck, Gillibrand has two.)
So when the time comes, they won’t even have to set up a new committee. They can just turn their leadership PACs into quasi-presidential campaigns. Some of them are probably already doing this. E.g., O’Malley still has his PAC from last time. The few candidates who might bother with an exploratory committee for 2020 will be those, like Julian Castro, who don’t already have PACs.