Hello FD,
Thanks for the response. That is what makes me most curious.
As I have undestood the case of Mmo is a question of certification. That Mcrit is in the area of 0.615M, so Boeing had to install the mach trim from that speed even though it maintains longitudinal stability without it.
Hence no restrictions on Mach-number with 'Mach trim fail' - light illuminated on the 738.
But the question of why there is an indicated airspeed - restriction during a Mach trim system failure still stands.