You’re are quite right of course. Two main weaknesses of these opinions polls, regardless of the methodology are:
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the outcome is based on the moment the question was asked and;
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the complex reality of voter behaviour is simplified by assuming that the change in support for each party from one election to the next is the same across all constituencies. Which fails to take into consideration the following:
Regional variations, new candidates and Issues and finally tactical voting.
So yeah in a nutshell these polls are utterly meaningless but in the absence of a crystal ball something is better than nothing, despite the crude nature of the something.
The last time this happened was in 1906 when Arthur Balfour lost his seat in the Manchester East constituency during the Liberal landslide victory.