There seems to be a lot of misconceptions in your post.
>"Additionally, running one more BGP session makes their network appear larger, so they benefit as well."
This doesn't make any sense. The number of "BPG sessions" is not a metric any ISP thinks about. A BPG session is just TCP connection. The number of prefixes an ISP is advertising might be something they care about however but for smaller ISPs that doesn't really matter as they are just customers of the Tier 1 ISPs and that just requires paying the Tier 1 ISP. It has nothing to do with the number of sessions or prefixes. I could have a single /22 prefix for my company and the Tier 1 ISP will still sell me the same service.
>"There's no rule that says ISPs should only accept /24 or shorter prefixes, and this is contributing to the depletion of IPv4 addresses. I would happily announce a /25 and return the other /25 to the world, but I must have a /24 to do it."
Well yes there is. Many ISPs will generally not accept routes smaller than a /24. They configure filter-lists that prevent those routes from being accepted. The reason for this is to reduce the size of the global routing table. Each prefix has to be stored in TCAM(memory) on a router. There have been many incidents where older routers couldn't handle the number of routes in the global routing table and caused them outages. See:
Further if you have a /24 you can certainly advertise 2 /25s to the same upstream ISP if you have more than one transit connection with them. In fact this is exactly how do traffic engineering.
>"It's frustrating that some ISPs make it artificially difficult for home users to obtain a BGP session, even when they're not asking for SLAs or dedicated connections."
This because home users absolutely don't need to run BGP! You just take a default route from your ISP and you're done with it. There is not point in running BGP if you are single-homed! You can't do any traffic engineering on it or influence the way upstream ISP route traffic to you. From the hobbyist point view it would be extremely boring. What can you actually do with it? View the global routing table? You can do this without ever running BGP by using a public looking glass server[1]. There is no incentive for ISP whose business is selling internet connectivity to cultivate a hobbyist community.
>"Additionally, running one more BGP session makes their network appear larger, so they benefit as well."
This doesn't make any sense. The number of "BPG sessions" is not a metric any ISP thinks about. A BPG session is just TCP connection. The number of prefixes an ISP is advertising might be something they care about however but for smaller ISPs that doesn't really matter as they are just customers of the Tier 1 ISPs and that just requires paying the Tier 1 ISP. It has nothing to do with the number of sessions or prefixes. I could have a single /22 prefix for my company and the Tier 1 ISP will still sell me the same service.
>"There's no rule that says ISPs should only accept /24 or shorter prefixes, and this is contributing to the depletion of IPv4 addresses. I would happily announce a /25 and return the other /25 to the world, but I must have a /24 to do it."
Well yes there is. Many ISPs will generally not accept routes smaller than a /24. They configure filter-lists that prevent those routes from being accepted. The reason for this is to reduce the size of the global routing table. Each prefix has to be stored in TCAM(memory) on a router. There have been many incidents where older routers couldn't handle the number of routes in the global routing table and caused them outages. See:
https://arstechnica.com/information-technology/2014/08/inter... and https://www.inap.com/blog/growing-pains-internet-global-rout...
Further if you have a /24 you can certainly advertise 2 /25s to the same upstream ISP if you have more than one transit connection with them. In fact this is exactly how do traffic engineering.
>"It's frustrating that some ISPs make it artificially difficult for home users to obtain a BGP session, even when they're not asking for SLAs or dedicated connections."
This because home users absolutely don't need to run BGP! You just take a default route from your ISP and you're done with it. There is not point in running BGP if you are single-homed! You can't do any traffic engineering on it or influence the way upstream ISP route traffic to you. From the hobbyist point view it would be extremely boring. What can you actually do with it? View the global routing table? You can do this without ever running BGP by using a public looking glass server[1]. There is no incentive for ISP whose business is selling internet connectivity to cultivate a hobbyist community.
[1] https://www.bgp4.as/looking-glasses