A pro-EU left, in the shape of the PS or not, isn't going to dissapear. And FI very probably won't establish themselves as uncontested leaders on the left in any case. In all honesty that would be expecting people to develop partisan loyalties that they really don't have; and the left will stay plural in a way where parties fluctuate up and down depending on the circumstances and the individuals. As you say, there are a lot of people who will go "well the mayor is doing a good job with the buses and the public library, so I'll vote for them again" or whatever. But the big tactical vote does probably signify to the left as a whole that people can be fairly forgiving with a candidate who isn't perfect and that they have all the reasons to actually sort themselves out and follow what the electorate is telling them.
And given the personality-based nature of so many French electoral entities over time, don't rule out the possibility of a future left-entity breakaway from the grand coalition assembled by Macron.
