While the backdoor and surveillance arguments are good, and the chips are very likely backdoored (if not deliberately then by undetected bugs) there are other issues with this closed source firmware.
Let's say another bug [1] is found that lets anyone remotely control your computer, but Intel becomes bankrupt, or just doesn't see it as a big enough threat to roll out a firmware update. You then essentially have a computer that you can't use, due to the fact it's not secure and anything done on it could be compromised.
Maybe not a massive deal for the average home user who would just buy another laptop. But let's say a large company buys 10k laptops all with an Intel chip inside it. Then Intel goes bankrupt, becomes incompetent (i.e. can not resolve bugs), refuses to upgrade firmware, or something else. When the next massive security bug is found (which is inevitable with all code, open source or closed) you are left essentially with 10k unusable laptops.
If the code was open sourced, the large company could pay someone else to fix the problem, or what's more likely is someone in the open source community would fix it for us.
The fact you have another processor running beside your main one, that has full access to everything you do without your permission, knowledge or ability to stop it should worry everyone. Even if there are no backdoors or bugs in the code right now, it's a very dangerous precedent to set that we buy hardware we can not control. Maybe one day Intel decides to put an expiry date in their chips, or some DRM to prevent you watching certain content without a license. These restrictions can't be good for society in the long term, can they?
But the biggest problem should be for large companies and corporations. They are putting the faith of their own business into Intel, which like all businesses could one day fail, big time.
1. https://www.intel.co.uk/content/www/uk/en/architecture-and-t...