Reasons for not Globally enabling Trace Flag 2453 - Cardinality on Table Variables
The main reasons not to enable TF 2453 globally are:


You have queries that deliberately used table variables for the one-row guess.
You absolutely must avoid recompilations as much as possible.


In many cases, you will still find that using other solutions (temporary tables, recompile hints) are better overall. Trace flag 2453 benefits a relatively small class of queries for which a table variable is the absolute best choice, but table cardinality is important without needing full statistics.

Somewhat related reading:


Interleaved execution for multi-statement table valued functions
Public Preview of Table Variable Deferred Compilation in Azure SQL Database
What's New in SQL Server 2019: Faster Table Variables (And New Parameter Sniffing Issues)