Piano plates can crack for different reasons.  One typical scenario
is failing of the pin block glue joint: the string tension pulls the
pin block forward, and 'voila'!  I own and fixed such a piano; it has
been fine for 15 years.

Every plate crack needs to be individually evaluated.  Some are benign.
A case in point are old Bechstein grands, which are known to be prone
to having a crack in a certain spot.  This does not seem to harm their
performance.

Many cracks can be repaired, be it through welding, brazing, bolting
on a splint or patch, or by adding some plate screws into the support
posts, as the case may require.  I find that restringing with a
recalculated scale, to a lower overall tension and tuning to A=440 Hz,
is preferable to tuning to a lower pitch.  It must be kept in mind that
a repaired plate is just that -- no one can guarantee a repair, and it
would be false to tell a customer it is as "good as new".  Some cracks
may be irreparable altogether.

A few years ago I sold a nice old piano and soon after delivery the
plate simply broke in an irreparable spot.  I replaced the instrument
with a different one.  (The customers got a better instrument, in
fact).  I invested money and time in moving and storing the broken
piano until I finally cut my losses.  I took out the action and keys
and gave the shell away for someone to do ??? with.  That's business
-- "Win some, lose some."

In a certain way, pianos can be time bombs.  Sure, a plate that
breaks is fortunately extremely rare, but it happened to me and it
could happen to anyone.  Hang around pianos long enough and it may
happen to you.

When a customer's instrument is in my workshop, i.e., in my control and
custody, I am responsible for it.  If there is a loss due to fire (or
a broken plate) for example, it can get expensive for me.  That is why
I carry insurance.  I can only recommend it.

Jurgen Goering