Bear in mind that the two roads to the west (Nonantum Road in Newton and Beacon St. in Watertown) have already been put on a road diet without any issues – they both narrow to 2-lane streets about 800 feet west of the rotary.
Of the three roads to the east, Parsons Street is already two lanes, and is a pretty minor street anyhow: its AADT was only 3940 vehicles in 2016. The second, North Beacon, is already a 2-lane city street south of the Pike; the northern part that comes into the roadway is in the midst of a 2-lane road diet right now
as part of the Birmingham Parkway lane reduction, which will extend to include North Beacon Street to the vicinity of IHOP. The busiest roadway is Soldiers Field Road, but even pre-pandemic, its volumes in this area (24617 AADT in 2018) were still low enough that
it, too, has been proposed to go to a two-lane cross-section.
Finally, remember that the rotary already has two sets of traffic lights (plus a third light nearby at Brooks Street), so functionally, this proposal isn't radically different from what already exists. In fact, the proposed design makes some movements considerably easier and safer by eliminating awful weaving movements behind both traffic lights that currently back up traffic inside the rotary (e.g. coming east from Nonantum Road to SFR eastbound, or SFR westbound to Brighton, or North Beacon to North Beacon in either direction). The traffic signals that are there now are also pretty old, so modern equipment could also make a difference in reducing backups.