This is not something that I think, but it's a FACT !
Almost all the medium and large companies out there, who are concerned about information and network security makes very serious effort to fight spam. You know why ? Email can be the most vulnerable network component that's exposed to the Internet in a company. An attacker has to normally bypass components like a firewall, a reverse proxy, a NAT, server firewall, user ACL, SELinux etc to gain access to a server/system (I am not talking about DDoS). Meanwhile, if the attacker sends a trojen infected file or a ransomware attached file to an employee of X company and if the recipient opens/installs the script - virus can either be spread to other systems or he can gain access to that machine and can wipe/lock data etc. Doesn't it make sense to you if a $ 3 Mil company goes for a $ 3k Cisco Iron Port to close the above said loop hole ?
Also, such companies will have well defined protocols/steps communicated with the employees on how to respond to unsolicited emails - types/actions etc. And, I don't have time to explain all related stuffs as well.
And about the resistance to spam from both GIs and TLDs - Like said above, companies that are concerned about their security will have more custom rules in place. Example : you send a 25M attachment to Google - no issue, you send a 6 MB file to a corporate IBM notes server - it rejects the message saying that your attachment size exceeded max allowed 5 MB (can vary). You send some emails to outlook.com or hotmail.com via a relay with all SPF/DKIM and DMARC intact - no issue, you repeat the same with Office360 (same vendor -MS) - you will be asked to verify the relation between your sending address and relay IP. I can pick a lot such examples where a corporate email domain being more aggressive than a free email provider. That's why, senderbase makes sense - senderscore makes sense - returnpath and its heavily priced certification makes sense.
Regarding deliverability - you may be right - cuz you have experience - and
@sendizo is also of the same opinion. I respect that. I may be having a different set of customers on a different arena. Anyway, as long as my approach works for me and yours works for you, there shouldn't be any problem for both of us
Peace !