Access is not a real RDBMS, and thus it sucks as a database, but it does interface nicely with the rest of MS-crappy products. (yes I take great pleasure in calling MS products crap🙂 )
Unfortunately, most managers and higher-ranked persons care only about how things look, not how they work. (wizard-lovers)
Informix is pretty much unbeatable as a database, but have you tried getting a report out of it with a point-and-click interface? :-)
If you have small numbers of records, you don't care about data-integrety and you have all the time in the world, stay with access.
If you need a reliable fast database, use MS-SQL
But, the easiest proof of which is better is in an experiment.
Create a database in access and MS-SQL, and fill both with 4 Million records.
Access will keel-over and die before you are done inserting, and MS-SQL will keep running and ask if that's all you've got. (ok, informix doesn't get interested untill you go over 10M rows, but that's not the issue)
If that's not a good reason to take your business-critical data out of access and into a real RDBMS, I don't know what is.