Finnish Winter War

Loss of 300 tanks was not low at all. I'm just speaking of facts: Finnish sources say of 300 (Koskimaa, Matti, Veitsenterällä, 1993, ISBN 951-0-18811-5, WSOY). Russian historians prefer not to speak of this battle in detail (who knows, why? ).
As for your question, why didn't "we" break through VKT line, the answer is obvious: because "we" did not want to, by such a price. I don't believe you doubt it was possible in theory
If this breakthrough was considered to be absolutely necessary, elite tank troops would not have been taken from Finnish front and sent to Estonia, would they?
As I wrote in the parallel thread, Stalin was a very practical politician. He might have not objected to conquer Finland by chance, but refused of it easily as he saw more important goals being threatened.

P. S. I feel some irritation in your question. Please, don't be angry: my sympathy is on the Finnish side, but I am a historian (though amateur) and so I speak of facts, emotionlessly (when possible).

 

I have found some additional materials on the subject, and some of them go from Finnish sources.
1) Lehmus K. Op. cit, s. 179-180; Wirtanen A. Salaiset keskustdut Lahti, 1967, s. 268; Wirtanen A. Poliitiset muistdmat Hels., 1972, s. 27; Seppälä H. Taistelu Leningradista ja Suomi, s. 272-273.
Lehmus, Finnish headquarters officer, tells that Stalin's telegram to Govorov (Leningrad front commander) was intercepted, in which Stalin tells Govorov that the war will end in Berlin, not in Helsinki, and insist on tank troops relocation to Estonia.
2) Govorov was sure he could reach Helsinki in a week with his former troops, but a different task was set: to pass a border to the depth of 8-10 km and set a defensive line Imatra-Virojoki in order to feign a threat to Helsinki and impel the Finnish government to drop out of the war.
Well, this task was not fulfilled, but the main goal was nevertheless achieved.