Yes, right now ARM + Android is about tablets and other mobile devices. But who said that they will stay there?

What keeps the wintel duopoly going is not the superiority of their technology, but all the software that run on the platform.

When there are sufficient number of quality application available for the ARM+Android platform, the new platform will be able to replace the old one by moving into PCs and laptops as well

This is a classic example of the kind of development discussed in "The Innovator's Dilemma" by By Clayton M. Christensen http://www.businessweek.com/chapter/christensen.htm

But even if we assume that most of the alien race is peaceful and ethical, it is still quite possible that at least a few of them are evil/cruel/sadist (just look at the serial killers in our own society). These evil few probably have access to the advanced technologies in their civilization too and can come to wreck havoc on Earth just for the fun of it (think of children using magnifying glass on ants)