Why does the Republican establishment support the prospect of 10 million new mostly Democratic voters?

It seems to me that 10 million new mostly Democratic voters would be a big problem for the Republican Party. But this is exactly what the establishment is fighting for! Right along with Hillary Clinton! As a casual libertarian observer, I don't understand it.

The Republican establishment hates Cruz and Trump, but those seem to be the only guys fighting against an amnesty program. Why is the establishment so set on amnesty? In the last Republican primary, these two anti-amnesty candidates received over 50% of the vote. So it would appear that many Republicans do not agree with the establishment on this issue.

It's almost as if the Republican establishment is trying to march the party into decades of obsolescence by padding the Democrats' numbers. Can anyone explain what is going on? Explanations from both sides of the amnesty issue are greatly encouraged! Thanks!