I second what Andrew says. I also recommend Karl Praust's 2004 paper (reference below) on the development of the class IX presents. According to Praust, the realization of /CnHC/ as [Cn̥HC] in class IX presents was avoided already in Proto-Indo-European in favor of [CnɪHC], with an epenthetic vowel, in order that -n- be non-syllabic throughout the inflectional paradigm, i.e. like the strong forms, which have the sequence /CneHC/. In other words, there was analogical pressure to keep the n a consonant, and the epenthetic vowel made that possible. The further development to -nī- in Indic would then be the result of regular sound change.
Praust cites the previous literature. Since then, there have been some further discussions: Lipp 2009:392–4 n. 97; Yoshida 2013; Kümmel 2016; and Gunkel 2018.
In my paper, I talk a bit about the development of the ī in class III presents of the type mímīte. The metrical evidence from the Rigveda suggests that we should restore some forms with a short i, esp. *mimihí (for transmitted mimīhí). jahimaḥ 'we leave' is attested in the Atharvaveda. Those show us the regular phonological development to short i (in accordance with Jamison's foundational study). The long vowels there are analogical, and I think it's attractive to have them be analogical to the class IX presents. If we follow Praust, then the chronology works nicely: the long ī in class IX would be in place early enough for that analogy to take place.
Gunkel, Dieter. 2018. Localization evidence for the restoration of Rigvedic *mimihi 'measure'. In Vina Diem Celebrent: Studies in Linguistics and Philology in Honor of Brent Vine, ed. Gunkel et al., 76–92. Ann Arbor: Beech Stave.
Kümmel, Martin Joachim. 2016. “Zur ‘Vokalisierung’ der Laryngale im Indoiranischen.” In Sahasram Ati Srajas: Indo-Iranian and Indo-European Studies in Honor of Stephanie W. Jamison ed. Gunkel et al., 216–26. Ann Arbor: Beech Stave.
Lipp, Reiner. 2009. Die indogermanischen und einzelsprachlichen Palatale im Indoiranischen. Vol. 2, Thorn-Problem, indoiranische Laryngalvokalisation. Heidelberg: Winter.
Praust, Karl. 2004. “Zur historischen Beurteilung von griech. κλίνω, der altindischen 9. Präsensklasse und zur Frage grundsprachlicher ‘ni-Präsentien’.” In Artes et Scientiae: Festschrift für Ralf-Peter Ritter zum 65. Geburtstag, ed. Peter Anreiter, Marialuise Haslinger, Heinz Dieter Pohl, and Helmut Winberger, 369–90. Vienna: Praesens.
Yoshida, Kazuhiko. 2013. “The weak affix -nī- in Sanskrit ninth class presents.” Münchener Studien zur Sprachwissenschaft 67:65–77.