If the candidate has met all the basic qualifications: leadership, service, sports, work/volunteer experience, competitive academics & PFE - no reason to delay. Doing EA gives the candidate first access to the coaches trying to fill their slots. More opportunity to be waitlisted rather than TWE.
Class size is not confirmed until EA is over so RD could be extremely competitive if the class size was small. 2021 looks like it might be another big class as the academy seeks to get to 1000 cadets so that may not be an issue. Even if waitlisted from EA, AO is known to fill up remaining RD slots from EA waitlist if no RD candidate is a match.
The golden rule for EA, is that candidate considers there is not much else they can do to improve to their application. Some in the forum have even stated that trying to achieve 20 extra points on academics is not worth waiting for RD.
Below are the 2020 EA average stats.
ref:
http://www.serviceacademyforums.com/index.php?threads/early-action-statistics.46339/
713 Applicants
196 Appointments
Of the 196 appointees to the Class of 2020...
53% AIM Graduates
Average SAT Scores: 678 CR 682 M 627 W (includes converted ACT scores)
Average HS Rank: 12% (includes reported and calculated HSR's)
Average GPA: 3.84 (all reported GPA's are converted to a 4.00 scale)
Average PFE Score: 230
45 U.S. states are represented plus U.S. citizens living in Belgium, Italy, Kuwait, and the U.S. Virgin Islands