What am I missing? With your system as well, people have
to change the way they send mail. They have to send to this
MX network. They can't send directly to port 25 of my (or
any) address. Thus, your system makes people change the how they
send mail as well.

Again, most of my spam is coming from virus-infested PC, spambots
etc, most on volatile IP addresses, who are sending directly to
my port 25 on my IP address. The only way to avoid these emails
is to drop the connections. The only way for real mail to get through
is if I accept connections from trusted relay servers. If other
people do the same, the only way for my mail to get out is to send
it through a trusted server.

All of this applies to both schemes.

I think the only difference is the way the trusted servers
operate. My scheme: non-spammers get someone to relay their
mail (I pay for this, but there are other possibilities). This
server rejects attempts to relay through it from non-customers.
Your scheme: the servers relay everything, but unknown stuff
is throttled. Spammers will realise this, and will send directly.
It will only get stopped if people accept email only from trusted
servers.

With your scheme, there are problems for people who legitimately
send many emails. For normal users, either the server has
to keep track of when the last email from a particular address
arrived, or it slows stuff down coming from unknown addresses.
The former is probably too much work. The latter will slow down
legitimate email from people without a static IP address.

With most people having a volatile address these days, one would
have to use SMTP authentication or the owner of the relay server
would have to know my current IP address. I think your scheme
would only work for someone with a fixed IP address.