So money can make more money?
well, currently we have the system with interest, which is nice when you have loads of money between 1975 and 1990, but else its useless.
so it would be nice if one could invest money in some way or another, also associated with risks. money sitting in the bank and being devalued by inflation is certainly annoying for anyone who is trying to save up a bit.
but regarding investments in other airlines... i am afraid it would make big problems:
there are airlines that have acquired so much cash, that they cant really use it anymore in their own market. but if they spill that money in "allied" airlines in other markets, this money can be enough to make turn make a mess out of smaller markets. so a limitation for such cash flows is needed. it already is a problem when airlines get subsidized without cash transfers; the rule against cash transfers is meant to reduce at least the worst symptoms of irrational carefree-spending management.