Isn't the £75 million option just a question of Chelsea protecting themselves against the (admittedly vanishingly small) chance that Mudryk demonstrates that he still has the abilities that led to them agreeing to pay up to £89 million for him more than 3 years ago? It would have been the height of madness on their part to agree a lower fee at this stage. Of course it is much more likely that by the end of this season Mudryk's perceived value will be less, very possibly significantly less, than the currently agreed £75 million and Spurs (and every other team) will have the option to offer a lower fee.
For the avoidance of doubt, I would much rather than we hadn't got ourselves in a position where we needed to turn to Mudryk as a left wing option. However, from what I've read it seems that we were pretty convinced that we were going to get Gakpo and that Ndaiye seemed a fairly bullet-proof alternative.