To be clear, NASA has an entire field center dedicated to rocket testing (https://en.wikipedia.org/wiki/Stennis_Space_Center). This is where everything gets tested now. You may remember the "green run" tests of the SLS - those happened at Stennis.

Stennis didn't exist at the start of the space race or Apollo. Marshall is colocated on Redstone Arsenal, a legacy of parts of NASA being bootstrapped out of the Army ballistic missile program. Marshall had test stands because that era of NASA (aka von Braun) sought to colocated engineering, prototyping and test.

One challenge with continuing tests at Marshall is that... it's actually really close to population centers. Large engine tests would be ridiculously disruptive. There are comments in the Ars article noting that people living in Huntsville could hear the demolition work.

Yes, the replacements for this equipment has been around for a long time. The Propulsion and Structural Test Facility was built at Marshall in 1957 and used for design testing of the Saturn engines, and by 1966 the A1 test stand was built at Stennis, to perform production qualification of Saturn engines. And unlike the PSTF, the A1 and A2 test stands at Stennis have been maintained over the years, and continue to be functional today most recently being used to test the new RS-25 engine design that the SLS will use when we are out of SSMEs.