Indeed,
Dies sind gute Studenten
is the correct way of speaking whereas
*Diese sind gute Studenten
while being fully comprehensible clearly reveils that German is not the 1st language of the person speaking.
All languages have their whims, and we just live with them. Asking "why" does often not help much. Or the explanation is so complicated that it would not help you in everyday communication. You have to know the correct forms "by heart" anyway.
But if you want to have a theoretical description, here is one:
When used in a sentence with the verb "sein" and the plural noun the pronoun refers to appears later in the same sentence (i.e. as part of the predicative), pronouns are used in their singular form (even when refering to something in plural) when they have the role of the subject in the sentence[1]:
Dies sind unreife Bananen
or alternatively, with less emphasis:
Das sind unreife Bananen
versus
Diese Bananen sind unreif
Welche sind unreif? Diese sind unreif.
Note that in the last example the plural Bananen does not appear in the sentence anymore, so now diese carries the plural.
[Subjects of the sentences highlighted in bold]
In your earlier question (see link in the comments above) you had the examples
Das sind Katzen.
Sie/es sind gute Studenten.
Other verbs
With other verbs use just the plural form:
A: Wo kommen die Bananen hin?
B: Diese kommen in den Keller. Die anderen da drüben kommen in die Garage.
But, of course, you also do not have the situation here of Bananen appearing later in the sentence, so there is nothing remarkable here.
And other copula verbs?
You were also asking for other copula verbs (bleiben, werden).
The question is, would we find examples like
Dies sind gute Schüler
where the plural noun is part of the predicate? This would require a more specific scenario because such situations do not emerge so often in everyday life. Let's try. Say, A and B are cooking a meal and are dealing with tomatoes.
A: Was machen wir mit den Tomaten? Machen wir die zu Ketchup?
B: Diese bleiben Tomaten. Diese werden Ketchup. Diese werden Tomatenscheiben.
Possible variation (less emphasis):
Die bleiben Tomaten. Die werden Ketchup. Die werden Tomatenscheiben.
However never:
*Dies bleiben Tomaten. Dies werden Ketchup. Dies werden Tomatenscheiben.
We see: bleiben and werden do not have the same effect on the pronoun as sein.
If we want to have a logical explanation: One could claim that efficiency (or call it laziness) is the motor here. Some unwritten rule like: Use plural only once. No need to use it twice on the same thing!
In Dies sind gute Schüler it is clear that Schüler is plural because the noun has that form. Whereas in Diese werden Tomatenscheiben the plural of Tomatenscheiben does not tell us anything about the nature (especially here the number) of what is meant by diese. There could be only one tomato which is going to be cut to slices. So here plural is essential for understanding whether we speak about one or more tomatoes.
I don't know if that's really what is going on in a German mind, but at least there is some plausibility to that explanation.
[1] Sorry, I admit that this is a very complex collection of conditions (3 times "when"). I suppose somebody with better training in German descriptive grammar could put it more briefly.
