If I'm not mistaken, I think that it's Jean-François Porchez, not Akira Kobayashi, who reverted to the curly form of italic lower-case f for the Sabon Next version. Tschichold's original design for this particular glyph may have been dictated at the time by the limitations of the Linotype hot-metal typesetting system, which did not allow for kerning letters.

I think it’s a good idea.
When italic /f starts a line, it goes way over into the margin otherwise.
I provided such /f’s as alternates in the Modern Suite.
Fedra has them, of course.

If I'm not mistaken, I think that it's Jean-François Porchez, not Akira Kobayashi [...] by the limitations of the Linotype hot-metal typesetting system, [...]

You are indeed correct. I re-checked and found that Sabon Next was not done by Kobayashi but by Jean-Francois Porchez.
Are you saying that this sword-like f is not a stylistic choice, but a compromise for the technology at the time?

When italic /f starts a line, it goes way over into the margin otherwise.

Good idea! I guess I have just focused on the stylistic side other than the practical function.
But, are they stylistically bad because a straight end is not most italic small f's are hand-written (viz. not following what a serif glyph should conventionally behave)?

Another Linotype method of improving fit was to rotate this character counter-clockwise a tad, but that looks more strange and obtrusive.

The whole Sabon project was indeed a matter of compromises : the brief Tschichold received asked for a Garamond-style text design suitable for handsetting, Monotype and Linotype hot-metal composition — three different systems with hugely varying technological constraints.
In my opinion, the fact that Tschichold managed to overcome all these limitations and to answer the brief in such a masterly fashion makes the original Sabon one of the greatest typographical masterworks of the twentieth century.

When the technical constraints that Sabon faced are mentioned, I am always reminded of the typefaces designed for the IBM Selectric Composer. All of them faced the same constraints as Sabon: as with Monotype, the widths of the characters were according to a unit system; as with Linotype, the italic and the normal (and also the bold) were the same width.

For comparison, here is Journal Roman, which I've thought of as the Garamond from the set of faces they had (although, looking at the bold, it also reminds me of Perpetua). This is from a scanned version of IBM's catalogue on the web, and the scan isn't the greatest, but it seems to be passable as long as one ignores the artifacts in the background.