Not voting in France's crucial second round is irresponsible

I can't for the life of me understand why French voters - or any voters for that matter, but especially French voters given the format of their elections - would abstain or vote "Neither" or otherwise spoil their ballots.

In the last French presidential election, back in the naive and innocent days of 2017, some 4 million voters voted blank or otherwise spoiled their vote, and another 12 million abstained completely (about 25% of the total). Those 16 million voters together represented substantially more than the number that voted for second-place Marie Le Pen (10.6 million), and would have technically been more than enough to have swung the election in her favour (although the majority of those abstentions are actually unlikely to have been right-leaning voters, and are more likely to have been leftists whose candidate was eliminated in the first ballot).

This year, centrist Emmanuel Macron is facing off against far-right Marine Le Pen once again, but Macron is unlikely to have as easy a run of it as in 2017. In the first round of voting, Macron garnered about 28% compared to 23% for Le Pen, with the far-left candidate Jen-Luc Mélenchon a close third with 22%. So, Macron is only slightly ahead of Le Pen going into today's  second round of voting, and how the non-Macron/Le Pen supporters vote is crucial.

You might think it a slam-dunk for Macron as all those who voted for Mélenchon in the first round would surely prefer Macron to Le Pen. But apparently a large proportion (around two-thirds, from polls) of them are planning on either not voting, or voting "Neither" in "protest" (in some ill-defined way). This seems to me a petulant, childish reaction. If, by not voting, they allow Marine Le Pen to sneak in, they are not going to be happy, and they will have doomed France to 5 years of unpredictable and extreme right-wing politics.

Surely, Macron is the better of the two evils, however much leftists may dislike him. So, wouldn't it be better to hold one's nose and vote for the lesser evil, rather than allow the greater evil to prevail? What kind of protest would that end up being? Not voting seems to me just irresponsible, particularly in this situation.

UPDATE

Well, Macron won, reasonably convincingly - sighs of relief all round - although not as convincingly as five years ago (58%-42%, as compared to 66%-34% in 2017). And yes, abstentions were high, at 28%, but enough Mélenchon supporters obviously saw reason and transferred their votes to Macron, rather than allow Le Pen to have her way.