The difference between the EU/UK (plus some other jurisdictions such as Israel) and the US is that legal protections exist which afford mandatory rights to passengers. The downside to this is that airlines will generally only pay what they legally have to and not make additional service recovery gestures.

In the US, you're probably a regular DL passenger. Potentially faced with competition and not wanting to lose you, when something goes wrong they throw a voucher or some miles your way. In situations where no legal redress exists, for example no catering provided, airlines in Europe will typically do the same. But if you're a one-time DL customer flying in First on a domestic - they likely don't care if they downgrade you so you'll just get a fare difference refund.

Now in your specific case, there's an argument under the legislation that you are entitled to base your reimbursement percentage on the total distance between AMS and ATH. That would push you into the 50% category of >1500km (but <3500km). Most airlines unsurprisingly take a different view and apply the lower figure - this has never been tested in court to the level of setting a precedent so you can't rely on it.

You were only downgraded on one sector, so you have to apply that percentage of your total journey. So what you're owed as a maximum is 29.8% x 50% = 14.9% of your original payment.

DL has probably already given you that.

In many cases with the lower percentage figures, the difference in fare (miles in this case) is more than the mandatory reimbursement. Here I doubt there's much in it. Realistically, it's probably not worth pursuing this any further.