Regardless, they're not "just fine." They're unlikely to be a playoff team, and have many years of probable rebuilding ahead of them, barring Jameis suddenly becoming elite with Payton's coaching. I don't know what else you mean by "just fine" unless you think I thought they or we in the future are going to be barred from playing in the league because of a cap situation.
My point is, we're bottom five in how full our roster is, we don't have a particularly large amount of talent on the team, and we have very little money to handle that problem with the looming contract on the horizon.
Also, let's take a look at round 3 RBs and how great they've been in the past six years, to see if "a top RB can be a round 3 pick next year." I'll star the guys making real impacts.
2021:
Trey Sermon - TBD, but won't start
2020:
Ke'Shawn Vaugn - 3rd stringer
Zack Moss - Maybe the starter? He's hurt though.
Darrynton Evans - Will barely see the field
2019:
Darrell Henderson - Only the starter due to injury, was replaced after year one
*David Montgomery - The first example of a good RB on this list
*Devin Singletary - Probably more credit than he's deserved given he's in a timeshare
Damien Harris - Has potential but was a complete wash year one
2018:
Royce Freeman - Non-factor to probably get released or traded, team signed a pricey FA
2017:
*Alvin Kamara - Legit star
*Kareem Hunt - Kinda half of a star deserved here because he let his team down
D'Onta Foreman - Non-factor
*James Conner - I'll give him a star but he was a non-factor as a rookie and then mediocre besides 2018
2016:
Kenyan Drake - Some flashes but ultimately not a starting caliber player for Miami (or us)
C.J. Prosise - Non-factor
So... out of 15 guys in six years, five guys have been legit players. Only 3 have had 1,000 yard seasons (although Kamara deserves a shout out for his passing game accomplishments).
The success rate isn't as high as you seem to think. It's not just a plug and play scenario. Especially when you consider that the successful rookies in that list are basically just Hunt and Kamara.