VOO is up since the iRobot acquisition was announced, which would have resulted in the effective price increasing over time, the opposite of what they want.

Nasdaq is ~ flat since then, which also would not capture the economic dynamic desired (some tech valuations have fallen). The Nasdaq Tech 100 does an even worse job at capturing the dynamic: it's up nearly 50% since the deal was announced.

You'd have to use something a lot more specific, like a basket of specific stocks or some very small sub-index. But then a smaller basket could have a lot more volatility that may be endogenous to a small set of companies, and someone has to get paid for that volatility.

This isn't totally uncommon. Companies just use their own shares instead of VOO.

This type of structure creates risks around ability to pay which a well advised seller would try to avoid. You could design something which would mitigate that but it would likely end up costing more for the buyer to set up and run anyway.

In private deals (one where the company being sold is not directly listed), the traditional way to solve this problem is via earn-outs.

In both public and private deals, using the purchasers' stock as the acquisition currency is also relatively common.

It’s baffling they don’t do it.