I'm surprised this is going forward. One of the benefit line items for the DRL/OL is that you get to avoid doing this work. $1 billion dollars gets you half of Vancouver's Canada line: we're spending that to get a couple staircases and not improve rapid transit coverage by 1 meter. Why not put that money towards extending the OL to Sheppard and shave off another couple thousand pphpd on Yonge traffic?
Notice that the new platform opens 2 years after the planned opening of the OL, which means it won't come until after much of the transfer traffic has been diverted.
Also, I find it ridiculous that they spend $1 billion on safety upgrades to this station and don't include PSDs??? If you're spending the better part of a decade and a billion dollars rebuilding this interchange for safety reasons, then I would expect barriers to the tracks to be part of the scope. Unless they're planning to shut down Yonge-Bloor again just a few years later. Line 2 is supposed to be done resignalling for ATC by 2029 since the SSE will be ATC-only.
They have no choice - OL won't change the fact that the Yonge Line extension will bring in more riders.
(I see @Northern Light had just said the same thing)