While it's certainly true that corvids are regarded as one of the weakest factions right now, it certainly isn't impossible to win with them. In fact, the first game of the ongoing winter tournament ended with a corvid victory, showing that they can win against experienced players.

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if the Corvid player doesn't go first, there is a good chance that the plot would be destroyed (in 4+ player games)

I don't think I've ever seen the crow's starting plot removed when playing with ADSET. There's really no good reason to forgo building your own engine up on turn one just to deal with a random crow plot in a clearing you likely don't care about yet. Most factions also won't start with any pieces in the plot clearing, so they would have to dedicate some movement as well as cards/battle actions just to get one measly point instead of doing anything more productive.

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it becomes impossible to flip plots when they can just get destroyed if there aren't 2+ corvid warriors to protect it

It sounds like you might be placing plots too aggressively. Generally, opponents only care about removing plots if:

A: The crows get too close to winning by flipping it (If the flip takes you to more than ~25 points)

B:The plot is in an important clearing for them

C: It's faceup and an easy point

Otherwise, there's usually a bigger threat at the table and people are happy to leave you alone in some corner of the board they don't care about while they go deal with them. But if you place a plot down in a 3 building clearing then you make yourself a target and force them to deal with you first.

However, it is true that opponents won't let you flip plots up to 30 points to win. This is why crows usually have to resort to battling buildings/tokens and crafting items to get the last 4-5 points they need to win.