Employees should have the same access to liquidity that founders get, but that's orthogonal to the 409A. The reason we want to keep the 409A low is so that when the company is valued at, say $100m, the common shares are much lower, say, $20m (even though they make up a majority of the company's shares!). Now when we hire that amazing person and offer them 1% of the company, their options have a strike price of $200k but already an expected value of $1m+.
Really the games we play with 409A valuations are a reason for founders to limit their own liquidity, too. Outside of a few famous examples like Google and Facebook, founders and employees have the same common shares, and it's in both their interests to keep the common share price low while building the company. If founders are selling shares and not inviting employees to participate, it isn't because of the 409A. Those founders are just assholes.
It's comical how much of modern startup compensation is gymnastics around taxes.
My understanding is that the most basic problem is if the startup gives you stock that's viewed as income and you will need to pay tax on an illiquid asset, and ISO/NSO/RSU/409a's are resulting from various elaborate schemes to give you stock-like upside tomorrow without having to pay taxes on an illiquid today.
In some ways that's true. Taking your list in order:
1. ISOs do have special tax treatment, but it only comes into play if you exercise them and the shares have increased in value (and it gets complicated if they've increased so much that they trigger the alternative minimum tax). Good companies though will let you exercise as soon as you join, so there's no tax at all until you sell. ISOs do nothing in that case.
2. NSOs are basically just standard call options. Most companies still force you to exercise or abandon your options after leaving, but good companies will convert your ISOs to NSOs and give you 10 years from the original grant to exercise them.
3. Startups doling out RSUs use triggers instead of handing out the shares directly (a taxable event), which is absolutely a tax dance. You also need to read the fine print on these, because some companies are evil and set it up so that you lose the RSUs if you leave.
4. The 409A is just a process for valuing the company. Keeping the value of the common shares low can be useful for taxes when the company uses options (less likely to trigger the alternative minimum tax), but that's not really why we do it. When a company uses RSUs, on the other hand, pricier is often better. Which is the natural progression of things anyway. The earlier a company is, the bigger the delta between the common and preferred share prices. Later on they converge, and when the company goes public, the preferred shares actually convert to common.
Really the games we play with 409A valuations are a reason for founders to limit their own liquidity, too. Outside of a few famous examples like Google and Facebook, founders and employees have the same common shares, and it's in both their interests to keep the common share price low while building the company. If founders are selling shares and not inviting employees to participate, it isn't because of the 409A. Those founders are just assholes.