The problem that usually happens when cheap bearings go bad is that the race inside the hub gets ruined too, and it's not economical to have a bike shop lace a new hub into a cheap wheel. I don't know about where you live, but in the US a bike shop can get a new cheap wheel for about what they'd have to charge just for labor for lacing a new hub into the old rim.
You can always replace the wheels with something better later, that's not the major problem I've seen.
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The problem with (some) dept-store bikes that I've seen is that the crank and headset bearings don't have dust covers. The crank bearings are under a far amount of stress when used and water gets in there and ruins the races and race seats, and then you have no way to fix that. With the headset bearings, many times the lower headset bearing is left open, and if you don't have a fender, when you ride in the rain the front tire throws dirty water UP into the lower headset bearings, ruining them.
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