The Window of Opportunity concept refers to the time when it is best to invest and deliver on a new idea. Too early can be a waste of money (WiMAX in hindsight), too late leaves you with not much to gain (current). It will be quite expensive to build out 5g, which is what Verizon will do to differentiate themselves in the market and to justify a higher price. Could there be other windows of opportunity for Sprint beyond this possible merger? Yes, but Sprint will have to wait for another carrier to stumble and also Sprint would hopefully be able to pay off more debt beforehand so they would have the capital (or better access to it) to seize such a new opportunity. Will Sprint immediately die without a merger? No. Sprint could continue to grow but at a much slower pace, although it might also slowly decline. Sprint would likely continue to look for new partners.
Vast additional sums of money would be needed to build out 5G in many major metropolitan markets in a significant fashion (ie noticeable on a daily basis by many people). Additional large sums are needed to make everyone aware of this (marketing).