As some here have already pointed out, it would be utterly stupid to sell CHO now. Put aside Chelsea / Homegrown / He's one of ours sentiment for now. From a purely business perspective, here are the possibilities if CHO doesn't sign in 6 months:
1) He plays regularly for the next 6 months. Does reasonably well, proving he is PL ready. Come summertime, he still refuses to sign (for whatever reason). Still with 1 year on his contract, if we list him for the current bid of 35m, there will be suitors. Possibly can ask for more given today's inflated prices for high potential youngsters.
2) He doesn't play at all in the next 6 months. Refuses to sign in the summer. Still with 1 year left, given previous hype, I would say you would get offers at 25m (30% discount). Might even have more than 1 interested party in the summer, which would increase the final price as well. If Bayern are interested now, there will be a market in 6 months time.
3) He plays regularly for the next 2-3 months. Does poorly and is benched. Or sustains a minor/medium injury and loses his place. Refuses to sign in the summer. Given his previous levels of hype, at worst, there will be interest at 20m (about 40% discount from current 35m). This would be the perfect price for someone like Dortmund who can develop him in a slightly "easier" league, and then sell him on again for 60m+
4) He plays regularly and gets seriously injured (Ligament, broken leg) (knock on wood!!) In this case, he'll probably sign with us, or maybe even lose the Chelsea contract as well. It'll be a long road to recovery, but with his technique, even if he loses his pace, there's still a fine player there. May have to learn a new position.
Scenario 1), if this happens, he most likely would sign with us anyways. And even if he doesn't, we're looking at the same transfer fee, or possibly more, than 35m
Scenario 2) I don't see this happening given our current Winger situation. Sarri also seems to have changed his tune, and I think we can all agree, would rather see CHO than Willian at this point. And if he really doesn't play, we're looking at a 10m "loss", which is nothing in today's transfer world.
Scenario 3) This could happen. In my mind, a "loss" of 15m is worth the risk for a potential superstar level player
Scenario 4) The absolute worst case and probably what the club is most worried about. But you could say the same thing about every expensive signing we make. The only difference I can think of is that without an open market transaction record, perhaps insurance companies would only value CHO at a few million pounds vs 35m? Hence if he has a terrible injury, Chelsea would only get insurance payouts based off of a much lower valuation.
If CHO refuses to sign in 6 months, scenarios 1-3 would still be considered a WIN by Chelsea in my book. Scenario 4 would be a LOSS, but with such a generational talent, risking 35m (aka 1 Thirsty Danny) seems like a no brainer to me.