But will this affect or shorten BTC initial end generating date ?
No difference, bitcoin mining difficulty move to correct this fact :
https://bitcoinwisdom.com/bitcoin/difficultyeven if the 50% of mining product shutdown, bitcoin difficulty decrease dramaticly to correct this.
This is not correct. If difficulty were to increase by 100% and them remain exactly the same forever it would mean that it took 1 week instead of 2 to reach the difficulty to change (and then it took exactly 2 weeks to change thereafter) therefore the date that the block subsidies would stop would be one week less.