Biggest MS mistake (ARM and i5)?

Everyone is smart AFTER the events, but I think the biggest mistake of Microsoft was to bring ARM and i5 Surfaces.

The best solution, IMHO, was to bring one single device - Surface with Atom - a ~10mm tablet running all basic legacy apps with 8-10 hrs of battery life. With Wacom stylus. Sounds good?

And in real world we have Surface RT with no legacy apps support and without a stylus. On the other hand we have a thick and heavy Surface Pro with poor battery life.

The hardware (processors) is not there yet, Microsoft. You can't use i5 in a tablet. Or actually you can, but don't expect good battery life or sensible size/weight.

What are your opinions, gentlemen?