It’s not an either-or situation.
a) if they are in mortal sin, they should not receive, but should make a spiritual communion.
b) enough time should be set aside outside of Mass for those who need to go to confession to go.
Some norms from Pope Paul VI:
“4. Local Ordinaries and, to the extent that they are concerned, priests are bound in conscience to see that the number of confessors should not become reduced because some priests neglect this noble ministry (cf. Presbyterorum Ordinis, 5, 13, Christus Dominus, 30), while involving themselves in secular affairs or devoting themselves to less necessary ministries, especially if these ministries can be performed by deacons or suitable lay people.”
“9. In order that the faithful may easily be able to satisfy the obligation of making an individual confession, let care be taken that confessors are available in the churches on days and at hours that are convenient for the faithful.”