But how can we get more and more real world merchants accept BTC with a transaction speed of 3~7 per second?
We can't.Bitcoin will remain as a "digital gold"-store of value.
No cryptocurrency will become widely adopted by the merchants anytime soon.
Actually there is only a small subset of merchants that need this "3 secs transaction". Many things can be bought with a day long transaction just fine, its just the typical coffee thing. In general, anything bought online can wait a day or so perfectly fine.
Gold is actually more cumbersome, if there was a merchant willing to take it directly, you would need to take it there which is risky, and the merchant would have check it to see if its not fake or low quality, weight etc..
In "good" days bitcoin transactions confirm in under 1 hour, which is fine for most things. In "bad" days it may take a day or so. I haven't seen the early 2018 mess repeat again.
Offchain "micro" transactions should also help reduce traffic somewhat for onchain transactions. Its not perfect, but it does fit that need. Yes i know LN transactions eventually get consolidated onchain. Perhaps there was a better way to design this.
Beyond legal/accounting reasons, i don't see why most merchants don't just adopt Bitcoin, and some other altcoins to look "trendy/fashionable".
The way i see it, once the major wallets adopt LN, it will "just happen". But we will see how this gets implemented and if it is user friendly enough. To me, LN transactions should be exceptional, not default.