Will apps update free forever?

Depends on the dev, doesn't it?
I have seen more apps that used to be for money, that are either reduced in price or free.
Why worry?

 

You will never be auto updated to a paid app, so you'll have to choose to pay for it.

If an app you use becomes a paid app, simply do not update.

 

If you buy an app, updates are free.

But, what some devs can do is stop developing the app you paid for, create a new app that does the same thing called app+ or something and then you'd need to buy that to get that app.