Well, the suttas aren't 'new' either. Eleventh century paperwork isn't old enough? I'm imagining someone with something like a wine cellar for toilet paper. "Ah yes, a '77, a particularly fine year for bum wipe." Some parts of copies of parts of the Tipitaka I have had have been so pawed over by me that I have had to replace them several times already. Personally, I can't afford to buy books just to put them into some kind of time capsule.
I've seen questions like this raised a few times. It appears to be something suggested by people who haven't actually read the whole Tipitaka. Have you read it? Have the authors quoted read it? I've read it over, a few times now, to the extent that it is possible for me. Overall, this is not the sort of message that lends itself to being mucked around with. The results of doing so would be like introducing gross errors to our collections of fundamental engineering texts. Planes would start falling out of the sky, buildings would fall over, etc.. Things would stop working.
Given that the Buddha's teaching is no less relevant today. Given that these teachings and this understanding are no less practicable and verifiable today, suggesting that the Buddha's message has been tampered with must then necessarily lead one to conclude that the universe has likewise been tampered with to accord with the suggested textual changes. Otherwise, the Buddha's message would no longer accord faithfully to the observable realities of today. Given that the instructions are still efficacious and the truths remain no less testable and continue to be consistently observable it is much more likely that there have been few if any modifications of any significance.
So, in addition to reading the Tipitaka, one might ask such questioners, have you applied it? Such an approach will resolve these kinds of issues far more completely and expediently than any amount of speculation about any lost artifacts of the past ever could.
Unless I've misunderstood the intention of the Buddha's life work and he was not interested in the promotion of our liberation from samsara but rather in the advancement of our enslavement to trivia and mediocrity.

But whoever walking, standing, sitting, or lying down overcomes thought, delighting in the stilling of thought: he's capable, a monk like this, of touching superlative self-awakening. § 110. {Iti 4.11; Iti 115}