After all 90% of web apps are just forms with some validation and lists of things - a spreadsheet can do all that with greater flexibility as a bonus.
It wasn't easy, and there was a lot to figure out, but the end result is starting to look pretty simple:
I love the premise - will give it a go.
Speadsheets are great at a lot of things, but data validation doesn't tend to be one of them. In fact, I'd argue that's one of the main reasons to move off of spreadsheets: to make your data more structured.
It may well be possible to create a spreadsheet-like UI that is good at these things though. And I can certainly see that being successful. It'd be difficult to tradeoff flexible vs constrained though.
What limitations are you thinking of? I can go to Data -> Data Tools -> Data Validation and restrict to whole number, decimal, list, date, time, or string length. If that's insufficient, you can create a custom formula which has to evaluate to TRUE for the input to be considered valid. Regex isn't supported out of the box, but quite a number of string functions are, and there are readily available Regex user defined functions (VBA called via formala) available online.
You can also customize the error alert that would be displayed to the user if they try to input something invalid. I think it'd work quite well for many MVP/single page web apps.
It's what a sufficiently motivated incompetent person can do to your data with the tools.
Infinite flexibility in the tooling means infinite ways to mess the database up in subtle and/or irredeemable ways, and a formula in a cell or even a regular expression are not great ways to tidy up real-world data, you often want autocorrect, autosuggest, defaults and friendly error messages for bad data rather than just ERROR IN CELL G91. When you reach that level of complexity it becomes much harder to build something useful with a spreadsheet-like tool alone.
It is a really interesting idea though and for certain classes of data could really work well.
If that functionality existed and was easy to use, it could have been a very convenient option for businesses running their operations on Access database and looking to take advantage of mobile.
I know Microsoft has InfoPath and Forms, but those aren't Access, so they're not going to be as popular.