I was curious about this so I looked up the $129 Kent. It's not nearly as bad as I thought it might have been. I can't find any spec details, but it at least looks like a basic steel bike. That's a big improvement over a lot of what I've seen in big box stores in recent years.

A lot of the bikes I've seen at these stores have been designed in ways that I just couldn't understand -- bulky, bulging frames with components attached in all kinds of non-standard ways. In a lot of ways they just couldn't be upgraded.

If the new Kent is a basic steel road frame with a 1" threaded fork, a BSA bottom bracket, 130mm spaced rear dropouts and maybe even a 27.2 seatpost (as it appears to be) then the case for upgrading gets a lot better. You can replace things bit by bit as you see the need. Eventually you can even upgrade the frame and you've got a whole new bike, George Washington's Axe style.

The key, I think, is to always upgrade to parts that you'll actually want rather than parts that you think are somehow appropriate for the lowly status of the bike you're putting them on. I say that with the caveat that I think the suggestions like UN55 bottom bracket and Sora components are appropriate. If your ultimate goal is to have an Ultegra-equipped carbon wonderbike, then it's probably best to just buy one. But if you goal is to have a basic, utilitarian bike with reliable components, you can do it one piece at a time.

Of course, buying a new bike, even of the sort just described, is probably cheaper than upgrading one piece at a time, but if budget doesn't allow the new bike you can enjoy as much as you can afford in the short run and might eventually discover that you don't really need to replace everything after all. Also, if the new bike you upgrade to is an entry-level Claris equipped bike (nothing wrong with that), you'll already have things like nicer wheels, tires, saddles...whatever was important to you(!)...to put on it.