Already happened, bigly: this is a good part of the Mélenchon surge and, on the other - "social-liberal" - end, several PS leaders (starting with Valls who broke his pledge) are openly supporting Macron, leaving Hamon bled from both sides.

The législatives in June are another game entirely: this is where you need nation wide party structure support to pull it, like the PS or LR.

As I have argued, the main focus will be for the newly elected president to get a majority to support him or her, and there's little chance for Hamon or Fillon to make it...

The first effect of the presidential election might very well be an implosion, not of the PS or the LR, or both.

The FN has always struggled to find enough candidates, and some of them have been too obviously moron. Macron claims he'll run candidates under his own En Marche movement label, but he'll likely get support from Bayrou, some PS who already support him, especially if Hamon is soundly beaten, and possibly some "moderate" LR.