As for why I think mass adoption is necessary, I should probably qualify that a bit. I think it's important that the network not consolidate into the hands of a few large players very early on. I'd prefer it never did but that's likely a pipe dream. Currently, the hypothetical big players of crypto in the future are still small, because they require crypto to build the businesses that will make them big. Big players right now, in turn, are not likely to do anything very interesting at all with crypto, because they're already successful even without it, and are too entrenched in the financial system to risk upsetting it. So in a nutshell, my worry is that capping userbase too low too early will lead to a situation where the only parties with the clout to use the system are those with no interest in running it.
Let me address this part only as iCEBREAKER has done a good enough job at debunking the rest.
The network is already consolidated into the hands of only a few player. An irresponsable increase of the block size risks precipitating this concentration.
It is also important that you reconsider this myth that capping Bitcoin's
transactions is a cap on its
userbase.
Bitcoin, as is, can provide a refuge for the wealth of ANY person on the planet. It can currently accommodate an infinite amount of capital.
The distinction is important because reality shows us that actual Bitcoin users are by and large not much interested in transacting or spending their coins. A great majority of the coins in existence have been and should continue to sit pretty in their cold wallets for years to come.
Why should we expect this to be different for future users? If you believe, much like I do, that the next trove of adopters is likely to be institutional investors with an interest in Bitcoin as a commodity/asset then why should we be worried so much with the system's transactions throughput? Do you expect the Winklevies to go on a shopping frenzy with their 100,000 coins? The analogy with gold is tired but it remains true: it should be expect that most bitcoins in circulation will be stored in digital vaults and shall remain untouched for a long period of time.
All of this is to say that you should stop focusing on "mainstream" users and what you assume their interest in Bitcoin will be. We should expect these to be last in line in your mass adoption scenario.