It will be necessary whenever we're ready to extend the OL to Sheppard, which realistically should be today. MX themselves has acknowledged that the Yonge Line will not see major relief until the subway on Don Mills Road is extended towards Sheppard. We need to be having these discussions right now, unless we've decided we're done with the idea of Yonge Line relief.
If the OL is built as currently designed, and we immediately extend the line to Sheppard (as we should), we'll be in a situation where we will need expensive retrofits on a brand new rapid transit line. Heck, we might even be planning for those retrofits before the OL is even running (if we want the OL North to be under construction by 2031, we'll need to begin planning for the extension within the next couple of years). This makes no sense. I'm sure none of us would be too amused if we were today if MX was discussing expensive retrofits to the Eglinton Crosstown before that line has seen a single passenger.
The only situation in which lower capacity makes any sense is if we build the OL, and don't extend it north for the next twenty of thirty years, largely giving up on any hope of Yonge Line relief. In that case, we've failed at our jobs miserably.