Some other observations of why the whole constitutional system supports this as well.
Most court suits start on the state level. the federal government only has appellate jurisdiction (much abused today though). That is where legal tender matters, and will most commonly appear.
The seventh amendment specifies that suits over 20 dollars have a right to jury. It doesn't say "bills of credit" or "federal reserve notes", and dollar doesn't mean something the federal government made up - it was a well known piece of money at that time, and meant a silver coin of a certain size. And it still means it since we go by what was meant when the constitution was made.
The tenth amendment specifically prevents the federal government from assuming more power than it has. If there is any attempt to take a power not specifically given to it but the States - in this case to make legal tender - the tenth amendment already says NO.
And for the support of this Declaration, with a firm reliance on the protection of Divine Providence, we mutually pledge to each other our lives, our fortunes and our sacred honor.