I think that handwaves away the real reason why the Kievan Rus fell, it wasn't a state in itself but more of a confederation of principalities.
Yes, it was close to "a confederation of principalities". Very close.
For a foreign reader (predominantly 'western') a few comparisons might be made to better understand that system. (*And ye, before you say so, all the comparisons are faulty by definition.)
But anyway:
Kievan Rus was like the HRE,
but...
1) of course smaller and less populous, that's obvious
2) was (almost) homogeneous 'ethnically' - predominantly Slavic, speaking (almost) the same language and sharing (almost) the the same culture
3) all the princes (petty, middle and great ones) were
exclusively from one bloodline only - the Ryurikids. As the Ryurikids saw it - Kievan Rus was their mutual family property/possession; and the population somehow shared this point of view, that was a widely excepted conception.
4) with every passing year Kievan Rus felt it's religious 'particularity', especially with Byzantine waning away. (Orthodox Christian) religion was something which started to differ Kievan Rus from all other countries, 'us against everybody else' feeling, sort of.
My point here is that in some aspects (Kievan) Rus was as cohesive as HRE, and when felt threatened was able to field 'confederation' armies big enough.
- Could this 'confederation' be wanked? I am afraid no, except for some period of time to be united by a strong charismatic Ryurikid, and then after his death immediately coming back to it's 'normal' confederation condition.
- Could this 'confederation' survive for a long time? With the Mongols butterflied away - most probably yes, as it had the most resistant sense of unity, the same self-identity (even in OTL, being conquered by different foreign powers).
The actual wanking might happen somewhere in the XV-XVIIth centuries, but that would be a totally different story, and even if the name
(Kievan) 'Rus' had been preserved the entity would have been a different beast(s) entirely.