I am treating each of these as their own scenario
2008: NJ and KY
NJ because its the only race that year Dems won by less than 15% where Dems would probably have gotten the seat back in 2014, and KY because getting rid of McConnell weakens R obstruction, and there were no races Reps won by less than 15% where Dems could probably hold on in 2014
2010: WA and WI
WA because Dems definitely get it back in 2016, and WI because it probably allows Dems to hold on in 2016 and 2022, and Feingold was also really good
2012: ND and AZ
ND because Dems lost it in 2018 anyways, and Heitkamp was never all that good anyways, and AZ because it prevents Sinema who was a bad Dems from winning in 2018
2014: MN and NC
MN because Franken was a sexual predator and in a seat Dems probably could have taken back in 2020, and NC because it probably saves Hagans life and Dems probably could have held on in 2020
2016: IL and WI
IL because Dems would have gotten it back in 2022, and Kirk probably would have placed a meaningful level of votes against his party in the process too, and WI has the same logic as with 2010
2018: WV and FL
WV because Manchin was a bad Dem in a seat Dems would lose in 2024 no matter what anyways, and FL because Rick Scott is that terrible
2020: VA and ME
VA because Dems probably get that seat back in 2026 no matter what, and ME because it probably shores up Dems in being able to hold the seat no matter what in 2026
2022: CO and WI
CO because Dems probably get that seat back in 2028 no matter what, against a Rep who was a bit less bad than most, and WI because it probably matters the most going into 2028
If you want to get rid of Sinema why not have her lose in 2018?