1) That's 40 rockets launched in a matter a few minutes. You also have to count reloading time, which affects the amount of total ammo spent in a given amount of time.
Given 5 minutes, who would be able to fire more ammo, a MRL or artillery??
It takes a long time to reload a MRLS. The answer is artillery because while they may have longer rate of fire against the BM-21....they make up for it in reloading time, which is insignificant.
2) Yes, unguided munitions are cheaper but smart munitions can be made cheaper through cheaper/better/widespread tech and it is being worked upon.
3) Cluster bombs are used to destroy buildings in which rebels/terrorists might be hiding. You won't be dropping cluster bombs in a real war with a near-peer or peer adversary since dropping cluster bombs demands total air dominance.
4) In a real war with equal or near equally powerful rivals, smart munitions will be used. US doesn't use them because they don't need to, not because they can't afford to. I don't know if you know this but the budget of the Pentagon was almost $ 1 T this (or maybe last) year. It has mostly risen for the past few decades. Nothing is too expensive for them.
When they can fire 30+ Tomahawks (valued at million or more $ each) at a Syrian Airbase where they didn't even achieve their goal, I'm sure they can spend a few more $$ on smart weapons.
5) CM's are a lot more expensive than guided and smart arty pieces like I explained in #4. You only use them for high value targets. CM's can't be used as SAM's either like I explained earlier or used as counter fire to destroy enemy rounds in the air before they reach their targets.
6) In conclusion, guided/smart weapons are the future, they'll have longer ranges too. For example, the BM-21 in your video only has a 45 km range, Pakistani MRLS have even lesser range.
The US's new Arty Project (explained further up in the thread) can reach upto and beyond 70 km and it's gps guided. It'll destroy any MRLS before they can even get in range to fire back.