No offense or disrespect but what proofs does your elder brother have? Did he time traveled or what like that Reddit guy? Or did he secretly invented a quantum computer in hopes of mining all 21 million so quickly? Bitcoin would still exist even if all of it would be mined by 2140.
When it comes to the price of Bitcoin, we’re uncertain of it because we just don’t know what would be the next move of it like finding the next Elon Musk to pump Bitcoin to $100kish or so.
This is just the case of someone being unable of knowing when to listen to his brother and when not to do it, obviously as his brother he trusts in him and he believes in him, however as you say unless he is an expert on this field and he knows a great deal about trading and investing then his opinion is worth nothing, the opinion of most users in the forum is way more valuable as they know this market way more deeply and have some knowledge about trading, so the OP should not listen to his brother when it comes to this topic.