Current trending already shows that Android will overtake iPad in 2015 for 100%. With Microsoft in a mix, nobody knows. But unless Apple does something drastic, I believe 2015 or even 2014 will be a decline for iPad. Same way as Android slowly but surely overtook iPhone marketshare.
Source: http://www.idc.com/getdoc.jsp?containerId=prUS23371312
That highly speculative graph would be true IF Apple sticks with the same form factor.
That's what I said by Apples to loose.
If Apple offers more variants for different market segments, then they stay on top.