From my perspective, there is a significant difference between a home-grown 5 yr senior/grad student and a 5th year transfer from a D-1 school. While there are clearly many D-3 players that have the skills to play D-1 and may have chosen not to for reasons unique to them, many (not all) D-1 players have size and skill advantages over the usual D-3 player.
The other factor is that while there are many D3 players that have the skills to play at the D1 level, and may be comparable to many players that went the D1 route, those players at the D1 level will generally develop more simply due to playing against and training with, better players. It's really simple. Training with better players and playing against better players will make better players.
There are some exceptions, though. Some of the mid-major D1 conferences are probably comparable overall to some of the top D3 conferences such as the UAA. Many teams that play in the UAA, NESCAC, or any of the top D3 teams, really, could hold their own against many of the mid-majors. Even St Thomas, which transitioned to D1 from D3, has had some moderate success and there are plenty of D3 schools that were better than the Tommies when they made the move. It is true that the Tommies were 6-8-3 overall and 2-6-1 in the Summit conference but they have been in some close games vs some tough opponents and this is only their 2nd full year in D1. It was to be expected.
A team like John Hopkins with 9 graduate students has stacked the deck in their favor. 2 of the grad students (1 fwd and 1 defender) are home-grown, having played their entire career at Hopkins with the forward contributing 10 goals and 2 assists. The starting goalie and the backup goalies are both 5th year transfers from D -1 schools. The remaining 5 grad students are all 5th yr transfers from D-1 schools that play significant time and contributed 37 goals and 24 assists. But most D-3 schools have not followed the path of John Hopkins.
A national championship from JHU would not surprise me at all.
The other advantage that JHU has, and I do not fault the school or team for this at all, is that it has extensive and top-echelon graduate programs. Many D3 schools don't even have graduate programs or, if they do, the offerings are quite limited. I had this discussion with someone else and they argued that many D3 schools have graduate schools. That just is not true unless, as I stated, the offerings are limited. Major research institutions like JHU, Chicago, MIT, WashU, etc, will have more top-level graduate programs than the typical D3 liberal arts school.
I am curious about other sports and if there were appreciable numbers of transfers from D1 programs. I am aware of a few in cross country that went from D1 to a D2 or D3 as graduate students but am unsure of other sports.
I have comments in bold, above, as well.
The reverse of going from D1 to D3 is the athlete that goes from D3 to D1. This does not happen often but it does happen.
Gabbie Cesarone is a great example of this. She is an outstanding center back and coming out of St Charles HS in Chicagoland, she could have gone to many D1 schools and started as a freshman. Gabbie went WashU and had an outstanding career. In her sophomore year she was both a D3soccer.com 1st team All-American and a United Soccer Coaches 1st team All-American. In Gabbie's senior year at WashU she won just about every possible accolade you can imagine. WashU, like many D3 schools, had very limited competition in her junior year due to covid, unfortunately. She then went on to Minnesota to study public health as a grad student. For Minnesota, she started 19 of 19 games, led the team in minutes played, had 6 goals and 2 assists with 4 game-winners.
In another sport, cross country and track, Ella Baran is now at Colorado as a graduate student after competiting at Johns Hopkins. Baran is one of the top runners on one of the top women's cross country teams in the country. She took 7th place at the PAC-12 cross country championships and was Colorado's 3rd runner. Colorado won the team championship. This weekend she will be in the regional meet and will more than likely be competing in the NCAA championship after that.
Esther Seeland is another great example of an athlete that made the jump from D3 to D1. Seeland played soccer and ran track at Messiah. She started most games games throughout her soccer career at Messiah and won multiple National Championships in track. She also qualified for the Olympic Trials in the 800 meters. Seeland is now at Virginia and competed in cross country this fall and was fairly successful despite being more of a mid-distance runner. This upcoming winter & spring she should score a lot of points for the indoor & outdoor track teams.
I know that I am getting off topic and this is not the focus of the thread but I like hearing about D3 athletes that have found success at the D1 level.