“Siddurim have only been printed for the last few centuries. Before that they were handwritten and very expensive. Indeed, prior to the invention of typesetters and pulp paper, all sefarim were very expensive. From records of early printers, it appears that books that were heavily used frequently have no surviving copies (and with pre-1500, even fewer existed).”
Correct. But they still exist(the most prominent of which include that of Rav Saadia Gaon and the Machzor Vitri) – both of which do not contain “modeh ani”.
“If there is a record of someone introducing a tefilah, that answers your question. If it suddenly appears, that allows you to date it. Consider dating the origin of the prayers for the Israeli government and the IDF.”
Not sure what you’re looking for here.
“Do the temanim say “Modeh ani”? Does anyone not say it? In what early siddurim is it absent? Is it attributed by contemporaries to an author (as would be the case if it is recent, meaning the last 1500 years).”
I haven’t a clue if they say “Modeh ani” – I don’t have one of their siddurim handy, and I don’t know if there is anyone today that doesn’t say it. We are a giant blurb of different ‘cultures’ in yiddishkeit today where it becomes increasingly difficult to hold on to the mesorah that ones parents or grandparents (etc etc) held to. Just look at the yeshivishe sefardim and see how few today really stay to their nusach. Siddurim are no exception and nusach-wise have suffered several waves of significant change. Not everyone who wrote a siddur was a talmud chochom (in fact, as I mentioned some were – or are – maskilim, amaratzim, etc etc). Their changes for whateer reasons have made their way into our siddurim today. If illegitimate nuschaos have penetrated mainstream siddurim, what makes you think that other legitimate nuschaos haven’t penetrated mainstream siddurim?
And as I wrote twice, yes: “modeh ani” is attributed across the board to the Seder HaYom – Rav Moshe ben Machir.
There are several “entry points” throughout time which demonstrate the opposite of your claim:
1. Kabbalistic additions from the Ari
2. Kabbalistic additions from the Shloh (who was metaken the nusach of today’s Kabbolos Shabbos +/- ana b’choach – that’s debatable),
3. Maskilishe additions (noting most did not affect both nuschaos).
4. Ma’atikim who did and did not necessarily do the best jobs
5. Today’s matzav in E”Y where plenty is borrowed from different nuschaos depending on who authors the siddur and/or from the jumble of nuschaos from having come into a common living place (e.g. Nusach Ashkenaz never said “ein kelokeinu” after davening on a weekday until Ashkenazim swallowed in some of the minhagim of the Sefardim)
You have yet to bring one anecdotal piece of evidence to claim, “If both Ashkenazi and Sefardi sidduring have something, it probably dates back at least to the period that the Talmuds were being written (before the Ashkenazi/Sefardi split).”
Point me to a sefer, a teshuva, something that bares some level of evidence to what you’re claiming – because I really don’t see it.