Would they still be serving in the US Senate?

Harkin(D-IA) would have defeated Ernst(R-IA) in 2014 and win re-election in 2020.
Johnson(D-SD) is a tough call. 2014(Johnson(D-SD) vs Rounds(R-SD)) would be similar to the 2002(Johnson-D vs Thune-R). 2020(Johnson-D vs US Representative Dusty Johnson-R).
Rockefeller(D-WV) would have narrowly defeated Moore Capito(R-WV) in 2014. In 2020, Rockefeller(D-WV) would have narrowly defeated US Representative Alex Mooney(R-WV).

37)HI(Schatz-D)
38)CA(Padilla-D)
39)NY(Schumer-D)
40)VT(Leahy-D)
41)MD(Van Hollen-D)
42)CT(Blumenthal-D)
43)WA(Murray-D)
44)OR(Wyden-D)
45)IL(Duckworth-D)
46)NH(Hassan-D)
47)CO(Bennet-D)
48)NV(Cortez Masto-D)
49)AZ(Kelly-D)
50)PA(Fetterman-D)
51)GA(Warnock-D)
52)WI(Barnes-D)

37)CA(Padilla-D)
38)HI(Schatz-D)
39)NY(Schumer-D)
40)VT(Leahy-D)
41)MD(Van Hollen-D)
42)CT(Blumenthal-D)
43)WA(Murray-D)
44)OR(Wyden-D)
45)IL(Duckworth-D)
5 to 10 percent margin.
46)NH(Hassan-D)
47)CO(Bennet-D)
Less than 5 percent margin.
48)NV(Cortez Masto-D)
49)PA(Fetterman-D)
50)AZ(Kelly-D)
51)GA(Warnock-D)
52)WI(Barnes-D)

37)HI(Schatz-D)
38)CA(Padilla-D)
39)VT(Leahy-D)
40)NY(Schumer-D)
41)MD(Van Hollen-D)
42)CT(Blumenthal-D)
43)WA(Murray-D)
44)OR(Wyden-D)
45)IL(Duckworth-D)
46)CO(Bennet-D)
47)NH(Hassan-D)
48)AZ(Kelly-D)
49)NV(Cortez Masto-D)
50)PA(Fetterman-D)
51)GA(Warnock-D)
52)WI(Barnes-D)

They are not going to run.
Bolduc is a sacrificial lamb challenger.

1)VT(Leahy-D)37
2)HI(Schatz-D)38
3)NY(Schumer-D)39
4)CA(Padilla-D)40
5)MD(Van Hollen-D)41
6)CT(Blumenthal-D)42
7)OR(Wyden-D)43
8)IL(Duckworth-D)44
9)WA(Murray-D)45
10)CO(Bennet-D)46
11)NH(Hassan-D)47
Democrats are slightly favored to win
12)NV(Cortez Masto-D)48
13)AZ(Kelly-D)49
14)PA(Fetterman-D)50
15)GA(Warnock-D)51
16)WI(Barnes-D)52

Democrats will retain control of the US Senate in 2022.
Democrats are likely to win-
37)HI(Schatz-D)
38)NY(Schumer-D)
39)CA(Padilla-D)
40)VT(Leahy-D)
41)MD(Van Hollen-D)
42)CT(Blumenthal-D)
43)OR(Wyden-D)
44)WA(Murray-D)
45)IL(Duckworth-D)
46)CO(Bennet-D)
47)NH(Hassan-D)
48)AZ(Kelly-D)
49)NV(Cortez Masto-D)
50)PA(Fetterman-D)
51)GA(Warnock-D)
52)WI(Barnes-D)

RIP Cleland!

37)CA(Padilla-D)
38)HI(Schatz-D)
39)NY(Schumer-D)
40)VT(Leahy-D)
41)MD(Van Hollen-D)
42)CT(Blumenthal-D)
43)WA(Murray-D)
44)OR(Wyden-D)
45)IL(Duckworth-D)
46)CO(Bennet-D)
47)AZ(Kelly-D)
48)NV(Cortez Masto-D)
49)GA(Warnock-D)
50)NH(Hassan-D)
51)PA(Fetterman-D)
52)WI(Barnes-D)