Access Database is getting corrupt again and again

Good question - I'll bring it up in my upcoming DD meeting.

SQL express conversion is extremely expensive for complex DBs and provides a substantial performance reduction.

Class action lawsuit... Attorneys

While i understand what you are trying to convey, i wholeheartedly disagree.  My job, amongst many other things, is to advise clients when they are taking actions that risks their data.  This is one such case.  As long as you advise them of the risk, then they can't blame you or the product.  There are any number of way to maintain remote databases that doesn't risk the data, so being told to do something doesn't mean one follows blindly.

And if, you truly have no other option than to run an Access database via VPN then I urge you to have a bulletproof backup system in place that make multiple backups throughout the day.

"SQL express conversion is extremely expensive for complex DBs and provides a substantial performance reduction."

That is a valid observation. 

It all depends on the database.  The main issue is that often, applications (using an Access backend) aren't designed for client-server usage, as is the case of you start off with the intent of using SQL Server from the start.  So, yes, the conversion and further optimization can be time consuming, therefore costly, depending on the complexity of the application being converted.

That said you do gain on reliability, security, ...

I wish it were that simple, Daniel. In large corporations, there are levels of management you have to present your suggestions to & they approve. If they don't approve, you don't move forward, and you have to come up with other options. Also a key question from mgmt is usually "What's the ROI on this idea ?". If they don't like your answer, again you don't move forward.

Saying you won't use their VPN to log in because Access doesn't work well with VPN's is not an acceptable position. You have to work within your company/corporate environment, whether large or small. And as you know, some environments are quite political, so it's not just about your preferred technical option.