There are several cryptocurrencies which are regularly used for purchases. Just because BTC and meme-coins get the media coverage and speculation doesn't stop others being used for transactions.

Hidden amongst all your bullshit (which Bobr has dissected) you do have a few reasonable points

barely handle the transaction throughput of Visa

Firstly that's a non-sequitur, you don't need Visa throughput to be a currency, but scalability is a genuine issue in the long term. That will need a 2nd layer technology, probably using sharding or other approach for more parallelism and throughput.

Monero is confusing for average people

Generally true. Any self-custody solution will be more complex than just trusting a bank service, but substantial UI improvements can be made and some trusted simple apps will develop.

Monero is centralized

There is currently a small group of developers and researchers, but they are distributed geographically and not connected except by their involvement in the project. All hosting and communication occurs on multiple platforms, including distributed ones like this one. It's one of the most decentralized open-source projects around, certainly far better than BTC or ETH for example.

will not be allowed to be hard-forked

Bullshit, no-one can prevent hard-forks. Both source-forks and chain-forks can be created by anyone. That is a concern actually as bad actors can cause confusion.

community can’t decided unilaterally ... technical stagnation

It will be the reverse. Unanimity or a centralized foundation would cause stagnation. Monero has had forks in the past, and it will have more in the future. That guarantees technical innovation. Customers will choose their preferred version. It's a free market for currency innovation.

US hegemony is over. Get used to it.

In the UK as well. My bank is pushing the phone app, and making it difficult to use the website without a phone. I will probably change my banking provider soon.

I don't know of any shop or street/market trader that will "write down a card number" or accept a written IOU from a random customer. In Europe cheques are practically extinct. When electronic systems or banks fail, cash is the obvious and generally the only working solution "in 2024".

You should read about relatives trying to help victims of romance scams on reddit /r/scams. Generally its hard or impossible to convince some who is emotionally invested. I recommend you point out past BTC volatility and drops, and suggest he diversifies a bit if that's all his savings. He could put some in gold, some in shares, leave some (maybe 50%) in BTC. He will still win big if it "goes to the moon", and will have some protection from a crash. Often you need to make a mistake before you can learn this lesson. Eggs in baskets.

The US dollar has lost about 98% of it's value since 1971 yet people still use it in trade rather than gold or silver or a better fiat like Swiss Franc. Don't underestimate the power of inertia. Just because USD has inflation problems doesn't mean XMR is about to take off. It will take hyperinflation or a currency crisis and even then better-known cryptos like BTC or LTC will be the first choice for normies.

I'm curious how he passed on the seed words to you. Did he write it down, give you the password for a password manager, put the info in his Will, transfer to your wallet address, or give you a hardware wallet? Did he just trust you with access to his computer data before he died?

If averages are published by a reputable website, shops can choose to use the average rather than "spot" price. In any case as XMR is more frequently exchanged for more stable goods or assets, the natural actions of buyers and seller will moderate fluctuations. However if you create a trading platform to trade averages vs spot prices that will tend to increase volatility, as short-term speculators benefit from volatility.

KYC is being used by many businesses as a convenient gatekeeping tool

The more important point is that the state has the gatekeeping and tracking power, they can see your transactions and refuse access because of "suspicious activity". The business is just "complying with regulations". If it was a business decision you could just find another.

If you want to install & update a FOSS app that's not on f-droid there's also Obtanium https://apt.izzysoft.de/fdroid/index/apk/dev.imranr.obtainium

At some level it's the fear of backing the wrong horse. To feel safe you have to be a "maxi", and to be a "maxi" you have to deny anything else can work. Bitcoin is the first crypto most people find, and the "everything else is a shitcoin" message creates a self-reinforcing cult atmosphere. The fact that mot altcoins soon fail reinforces that.

Almost all bitcoiners are there for "number go up" profit, so the price volatility also creates a lot of anxiety. The ones that stick with it become very committed HODLers. It's a bit like an abusive relationship, the pain makes you commit more. Market psychology is fascinating, and is why emotional investors lose their money.