Do I think that all 3xxx Ryzen CPUs will be on 7nm Zen 2? No. I think there's a possibility that the R3s will still be Zen+, though 6c minimum.
That's a reasonable argument to indicate that they need not necessarily use Zen 2 for the R3 3000 series chips, but I think your argument relies on the reasoning that R3 is the bottom of the barrel.
However, AMD still has Athlon positioned below that (the 200GE for example) so there is a still a bit of a dumping ground for old parts.