Well it's not like it'll harm your cat but it's a bit like giving them junk food diet. It'll sustain them but isn't the best thing to feed them. I'd rather spend a bit more and give them the best in my price range. The applaws bags last me months with 2 indoor cats. You might also find you have to feed more grams of whiskas a day than you do with applaws. 7.5kg bags are actually not too big. I always have 2 on the go as I like my cats to have something different in morning and evening. When I was buying royal canin I'd have 2 x 12kg bags on the go (lol)
I have a healthy 17 year old cat who says different (in cat talk), we've always used whiskas. Considering 1kg costs £4, this is a good price. But where to keep a 7kg bag?!
Spend a bit more and feed your cat on something a bit more wholesome. Whiskas is full of cereal and filler ingredients which don't give your cat much nutrition. I stock up on Applaws when the amazon s&s discounts come around and get 7.5kg bags for around £22