Obviously you cant scale to Visa with onchain transactions and keep node decentralization. But it does not mean you cant increase to 6tps or 12tps right now and give users much better experience with Bitcoin so it is well worth while keeping node decentralization, not insignificiant and unnecessary as you say.
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If the first 3 tps are insignificant then so are the next 3 or 9. So, from where does your magically occurring putative "much better experience with Bitcoin" originate?
Let's investigate!
UX @ 3tps: pay a competitive fee (about 12 cents) to ensure priority processing even during peak hours' surge charges
UX @ 6tps: pay a competitive fee (about 6 cents) to ensure priority processing even during peak hours' surge charges
UX @ 12tps: pay a competitive fee (about 3 cents) to ensure priority processing even during peak hours' surge charges
Wow, an entire 9 cents of savings and at only the cost of sacrificing Bitcoin's interesting diverse/diffuse/defensible/resilient properties. Sign me up for that!
Your understanding of fee economy is wrong which is part of the reason you dont see advantage with inceasing number of onchain transactions to the next 3/s soon. Basically when you saturate the block longterm so the backlog cannot clear, the fees rise much more + you limit the number of people who can use Bitcoin. And because there is no working Bitcoin offchain solution yet to serve more Bitcoin users you basically say go people away to altcoins, Bitcoin is only for the few ones paying high fees.
Now if there are Bitcoin offchain solutions ready and working, it is time when you can afford to not increase onchain transactions/second capacity if there is strong reason to loose decentralization substantionally and still you can hope high part of Bitcoin user base will preffer using more Bitcoin offchain solutions instead just going to altcoins.
Thats big difference why it is necessary to increase transactions/second capacity now even by the small 3/s to give more time to the offchains solutions to be completed so people dont need move to other altcoins when Bitcoin become saturated/unussable and without working Bitcoin offchains solutions.