"if the price of steem would remain unchanged"

Yea.."if" that's the key point. Steem is inflationary and again, all else being equal, the price of Steem will tend to drift downwards. Having your steem locked into SteemPower, which does increase over time, will give you some protection from this inflation, but the protection is not complete. Let me quote the whitepaper (which I'm sure you've read) :

"long-term holders are almost completely protected from the dilution used to fund growth."

Note the word "almost". The protection is not 100% (usually) so again, if you are inactive, the default is to see your account lose value over time.