Iphone 5s , Battery Drain after IOS 11.3.1 Update

Dear All,

am using Iphone 5s with IOS 9.2.1,This week accidentally i update the IOS 11.3.1 , after Update everything goes wrong before OS Update , am using mobile end of the day without charged , but after Update 11.3.1 , am using mobile only 5 hours after i stuck with charger , after that only am searching internet for this IOS 11 issue, apple asking apologies regarding this battery drain problem on December 2017, now my first question is you know have problem in IOS 11 then why your giving IOS 11 update option to all Iphones ????, Second Question why your block IOS downgrade option, this is very serious issue ,in this case we need only solution not your apologies , why your block downgrade option ?? actually you need to block update option but apple doing inverse , ASAP enable downgrade option , this is one and only solution waiting for your response