Two Piano Concertos - Joined at the Hips??

I unabashedly bow to my kid brother, who is responsible for this particular contribution to my 500 or so preceding blogs:
He had recently gone to a performance of Tchaikowsky's 2nd Piano Concerto at Lincoln Center, and in his relating this to me, I suddenly realized that I had never discussed this work in any of my blogs.
This work, and another piano concerto, written by another master from Mother Russia; namely, Rachmaninoff, are in a rather arcane way, joined at the hips, in that from my view, they represent these composers, in the concerto format, that veer off in directions that result in compositions that do not rise to the levels of predecessors in the same format these composers had created.
Please understand that these are my views only, and some of you may not agree:
For me, the First Concerto of Tchaikowsky contains ideas and the development of these ideas that have resulted in this concerto's immense world popularity. The second concerto simply does not contain, for me, the spontaneity and sense of direction that the first is known for.
To be sure, the 2nd contains shards of the great composer, with its wonderful melodic output; sadly, I feel that the piece is, simply, too long, and therefore "short-winded," with resultant lack of spontaneity, an elemental requisite within the realm of what we call Greatness.
Finally, how many of you have heard the 2nd concerto more than you may have heard the 1st?
As for Rachmaninoff, his 4th concerto, for me, does not reach the heights of two preceding works in the same format. Why?
In my view, it may simply be that Rachmaninoff embarks upon different directions endemic to his first three Concertos - for instance, his being influenced by the likes of Gershwin and Art Tatum result in some 'jazzy' incarnations; very witty and deftly written, but a departure from the Rachmaninoff the world is so adhered to.
As a composer, I fully understand the import of constant learning and investigation; however(and ironically!)the composer who is not considered 'great' can afford to 'change' in order to see another way. But we know the great composer can only 'change,' by way of increasing knowledge of "how to work things out" more thoroughly the base material that only the great composer can possess. Salieri had wonderful ideas, but he simply could not "work things out" like Mozart could.
So when Rachmaninoff actually alters the base of his creative direction, he is simply no longer Rachmaninoff as the world knows him. Alexander Scriabin is the only great composer I know of who successfully underwent a metamorphosis from Romanticism to Mysticism without losing his identity.
And now you know why I think of these two works as being, as it were, "joined at the hips."