I understand that the more people that can run their own node supports decentralization, but if miners are sufficiently decentralized what is the threat of users not being able to run their own nodes?
Every time you check your wallet, your wallet is displaying information that it fetched from a bitcoin node (usually with other software running on top of it, like ElectrumX). If you absolutely trust the server behind your wallet, then as an end-user, it's not necessary to run your node. However, you might still want to run it, because you care about bitcoin and want to help new users/companies download the blockchain, make the network more resistant to DDoS attacks or, if you live in an authoritarian nation like North Korea, you might want to make it harder for the government to censor bitcoin.
Running a node is especially important if you run a bitcoin-related company. In this situation, you shouldn't be relying on an external service as you don't know if it will be down, hacked or have bad intentions.
Also, if you receive a large payment in bitcoin, you want to make sure you received it. You can check seven different bitcoin explorers but, depending on your level of paranoia, you might want to check your own node and verify that it has at least two to six confirmations.
Check out Andreas's video about this.
There is a popular saying in Bitcoin: Don't Trust. Verify.
Even in a world where miners are perfectly benevolent and can be trusted to maintain the network's rules (highly unlikely, but lets just pretend), the user still gains benefit from running their own node:
By running a node, the user can interact with the Bitcoin network in a completely sovereign fashion. As long as they are running the correct code, the user can verify the state of the network themselves, without having to trust any third party.
By running a full node, the user will not have to query any other nodes directly to determine their wallet balance. Analysis of the blockchain is profitable business for several companies, so the prudent user will take steps to interact with the network in privacy-preserving ways, so that their financial data will not be compromised by these analysis firms and sold to the highest bidder. Running the node over the Tor network can help increase user privacy greatly.
It is worth noting that running a full node does not guarantee privacy, there are additional considerations here which are outside the scope of this question. But in general, running a node will be a net privacy gain in pretty much every case.
The points above are very important: by returning sovereignty to the user, we have also removed the potential for abuse from third parties. The user is in full control of their funds, and cannot be censored or restricted in access to them.
At the network level, it is full nodes that enforce the rules of the network, so by having a robust network of nodes run by individuals, we see that the network becomes resilient to change (which is a good thing, if you want the rules of your money to remain constant).
If you do not run a full node, you must assign varying levels of trust to third parties, in order to learn your balance, send transactions, or uphold the rules of the network.