When Bitcoin network hash rate shrinks a lot because miners shut down their rigs, Bitcoin blocks will need more time to be found and Bitcoin network will need more difficulty epochs to adjust.
Only until the next difficulty adjustment. After that, it would take normal times to mine blocks.
How can you say, it doesn't ensure a price increase, I mean, after each and every halving, back in time, the price of BTC improved and reached new ATH or isn't it?
Yes, in every single halving we've experienced so far, the price would increase more than the block reward decline. That's probably why hash rate has skyrocketed comparably to previous halvings. But, that doesn't mean history will repeat itself. We can't make schedules based merely on Bitcoin doubling in price every four years.