Have you ever heard of Monotype Grotesque? Probably not. It's like Akzidenz-Grotesk-- it just kinda faded away for some reason. It was made by Frank Hinman Pierpoint in 1926 for Monotype Corporation. In 1982, Monotype was supplying IBM with bitmap fonts for a line of computer printers-- one of them used Helvetica which MT licensed from Linotype and the other used Arial (called "Sonoran Sans" at the time) which was made specifically for the project by a 10-person team at MT and used Monotype Gothic as a basis.
In 1990, Arial as we know it was made into TTF and licensed to Microsoft.
Really, it's Microsoft's fault. They called Arial "an alternative to Helvetica". If they hadn't said that then no one probably would have thought to compare them. By a co-incidence of universal proportions, Arial's TTF outline managed to almost match Helvetica's character height and width. No one at MT planned that or said "Let's mutate Helvetica!" They say that there's no such thing as a truly random occurrance but this really was.
So now it's just like everything else. You're either Sony or Nintendo. Pepsi or Coke. Burger King or McDonalds. Mac or Windows. Arial or Helvetica.
Even so, alot of computers have both typefaces. Mine does. All Mac OS Xs are. IMO both Arial and Helvetica are overused. Fortunately, Microsoft made the solution in 1997.
Microsoft Sans Serif is the perfect midline between Helvetica and Arial. It has similar features to both, but still enough new designs to make it interesting. The lowercase a manages to accomplish what neither of the older types could. So does the uppercase R. It's 100% computerised. No personality of any kind. Bland enough to work famously with any heading font.
So, is Arial a cheap knock-off? No more so than Comic Sans is a cheap knock-off of One Stroke Script.