First of all, I want to congratulate you for stepping out of your usual Marxist-Leninist comfort zone, which seems to mainly include Vietnam, the USSR, Hungary, Yugoslavia somewhat, Australia, and some Anglosphere labor politics generally, to include China [though I noticed you commenting lately on the 1989 Shanghai Soviet to be fair].
Still, I am a slow learner and feel a little whiplash or confusion about whether you are trying to make the situation better or worse for the Chinese Communist Party, and/or its military at this moment in history (the mid-1930s). I'll diagram out my confusion.
What if the Long March decimated the CCP’s operational units?
To me 'decimate' sounds like making the TL worse for the CCP, or at least its military, than OTL, since decimate sounds bad, like killing alot of, and in OTL, the CCP military took over the country only 15 years later. On the other hand, technically, decimate going back to its latin root does just mean something like "kill a tenth" not the "kill em all" it evokes.
the Long March was a historical “retreat” by Chinese Communist Party operational light infantry forces under GMD strategic/political pressure.
Good description - it certainly wasn't an advance, or a "growth" or expansion in a military sense.
It resulted in force preservation and “retreat” into Yennan as a “safe” area where CCP forces had to cooperate with actual Chinese communists.
I don't get the inclusion of the words I bolded. Were the inhabitants and supporters of the party in other CCP controlled Soviet areas in south and central China besides Yennan/Yan'an that existed before the Long March not filled with "actual Chinese communists" but with fake Communists, foreign Communists, posers, unwilling hostage populations or something more than was the case in Yan'an?
but what if the CCP had been decimated by the long March?
what if they had lost 10% of their forces in a retreat?
instead of 90% of their forces in a rout?
This makes it sound like you interpret OTL's Long March as a *rout* of the CCP military, costing 90%, which is the typical estimate of losses, and your what-if is, what if they lost much less, only 10%.
So, in the end, I think you mean your what if to be about "what if the CCP was better off?," but 90% of readers will take away the opposite meaning, like "what if they were all killed off?"
But what do I know, it's not my idea, and I'm not a mind reader, just a word reader.