There is one linguistic point that convinces me that this is the correct interpretation of 2:106. The Arabic word " خير " is used in both 2:105 and 2:106. The word means both "good" and "better" and also "choice" as an adjective (describing something chosen for quality). In 2:105, "خير" is what the disbelievers don't like us to get from God, and in 2:106, "خير" describes what God promises to bring if He abrogates a verse. Coming in two adjacent verses, I believe that "خير" is referring to the same thing in both verses which would be the Quran, and this supports the interpretation that 2:106 is about the Quran abrogating previous books.
Being linguistic myself
, I find that to be the crux of the evidence that 2:106 speaks of the Quran abrogating prior scriptures.
Of course it is possible that 2:106 also describes verses that had been gone from the Quran during the life of the Prophet (PBUH).
That I don't see, especially given your analysis of context and the repetition of the word "خير" in 2:105 and 2:106. Rather, the verse refers, IMHO, to scriptures that were entirely forgotten, such as the scrolls of Abraham.
Sheikh Muhammad Ash-Sha`raawi interpreted 2:106 in a detailed way
- He discusses, in detail and with logic and clarity, that different times call for different rules, and that is why God abrogates previous messages as time goes on. He also mentions that matters of faith do not change with time, and explains that when a new message abrogates the previous one, the rules are better because of changing times, but the matters of faith are similar, and that is how interprets "better or similar" in 2:106.
- He applies the same argument to verses in the Quran abrogating other verses. He counters the argument that there cannot be abrogated verses in the Quran because it would constitute 'change of mind' or 'failure to anticipate' by arguing convincingly that abrogation does not imply that, but rather it reflects an understanding of what different times call for and legislating accordingly.
I hate to disagree with the reverend Sheikh, may God bless his soul, but what debunks his theory is that times have changed greatly since the Seventh Century and no new scriptures were since revealed. How then can hundreds of verses be abrogated during 23 years but none during 1400 years?