It will not be a game over other projects but another "stepping stone" that different coins will face during the improvement time. If the lightning network is applied everywhere, I don't think there would be another hater of BTC that would say it's only suitable for a store of value type of asset but an actual currency as to what we want. If that happens, I think replacing or dominating the cryptocurrency scene by BTC is even more apparent.

Bitcoin in its current state is not capable of supporting world-wide adoption of Lightning. Other solutions will need to be developed.

Many altcoins are kinda useless anyway. However, I don't expect LN have a crucial effect on them.

And LN is not a perfect solution. It's a step forward, but it will have its opponents and its problems too. Even the fact that it stays that long labelled as "beta" is a bit worrying.
Many won't understand that it is normal that it needs coins locked and on-chain transaction for finalize everything (I don't know how to say it better), many won't understand why it doesn't work just like that from <insert-wallet-name-here>, many (will) cry that it creates centralization and basically works only with custodian wallets.

All in all, many will prefer altcoins, even centralized altcoins, that do the same job. And if some of those altcoins will work seamlessly for payments at merchants, maybe also allowing some sort of atomic swap with Bitcoin, they could easily become a strong competitor for LN.

The Lightning Network is said to be able to take the transactions per second figure of Bitcoin and other cryptocurrencies to unprecedented heights of at least 1 million transactions per second.

Compare that with Visa that does around 1,700 transactions per second.

'El Salvador' and Jack Dorsey will be pushing this technology and also we will soon have the book Mastering the Lightning Network by Andreas Antonopoulos, Olaoluwa Osuntokun, and René Pickhardt available on the bookstores' shelves. 

All that is working synergetically to make LN solve all the bitcoin scalability issues.

Usage of Lightning Network is rising every day with number of nodes (over 21,400) and channels (over 49,800) being at all time high, but I doubt this will solve all scalability issues with Bitcoin.
It is not so popular with regular people, more development is needed, and more merchants and websites need to accept LN payment to see the real potential and fix all bugs.
It's always possible to have some new mainnet scaling solutions in future, or some better alternative to Lightning Network.

Excuse me from being a LN newbie. Can't you open 1 channel funding 4 multi-sig addresses?

I'm not entirely sure what you are asking here - I think you might have your terminology wrong.

It is possible to open multiple channels with one transaction, if that is what you mean, although not yet commonly done. It is also possible for more than 2 users to open a single channel by using something called Channel Factories, which allows more channels to be opened and closed between all these users without requiring any further transactions to be broadcast to the base layer, but I don't know if this has actually been successfully done by anyone yet.

Sure, you can! I have recently opened two channels in a single transaction. As far as I know, LND, which seems to be the most popular implementation, requires the operator to construct such a transaction manually using PSBTs. That's probably why it's not so popular. C-lightning has a dedicated multifundchannel command which is fairly easy to use. I don't recall seeing a mobile wallet which would support this feature.

I might, but what I want to say is that even if we make all those assumptions you wrote, it's still impractical to open a channel for every future Bitcoin user. We'll probably have to move into another smarter way like using 1 multi-sig address for a family; each member could have their own balance in their phone, but they'd all spend from the same channel.

There's a lot of development in LN these days as I can see. I'll dive into the details this summer, because I've stayed behind. I have never setup a lightning node like LND and C-lightning; I'll start with these.

I think one of the drawbacks of a add-on type of technology like this.. is the complexity. People are already struggling to understand the basic concept of Bitcoin and now they have to over complicate it, by adding more things to it. 

Technology should never be too complicated on the "user" side... or adoption will be hampered. People want a wallet and they want to deposit and withdraw... that's it...     (Running Lightning Network hubs and all that nonsense is for techies... not for your average public out there)