AHC; More archaic Indo-European langugaes

Make the Indo-European language family much more archaic than IOTL, i.e. all major langugae families (Germanic, Romance, Indo-Aryan) are as conservative as Slavic languages (6-7 nominal cases, verbs conjugated for person and number, synthetic tenses predominant over periphrastic ones etc)
I’m not sure there’s any single PoD to accomplish this, or even could accomplish this. As I understand language archaism, would need all of these groups to remain isolated and without much outside linguistic contact for a long time, which seems very unlikely to me.
The Britannica suggests that loss of case and verb morphology is to some degree a consequence of sound changes (https://www.britannica.com/topic/Indo-European-languages/Characteristic-developments-of-Indo-European-languages) - <i>"In phonology, the most striking changes have been loss or reduction in many languages of final or unaccented syllables, and loss in several languages of certain consonants between vowels, often followed by contraction of the resulting vowel sequence ... As a result of the fact that much of the marking of Proto-Indo-European inflectional categories was done in final syllables, loss and reduction of these syllables have often had serious grammatical consequences. In the noun, loss of endings has generally led to loss or great reduction of the case and gender systems, while ways have generally been found to salvage the distinction between singular and plural...In the verb, where more endings originally had two syllables, loss of final syllables has had less serious consequences for morphology. Even here, however, some languages, including English, have totally or almost totally given up the marking of subject by personal endings. ".</i>

So, I guess, a world where more Indo-European case and tense were present would avoid such sound changes. But I'm not sure how much the languages would sound much like the extant families we know of.