I am developing a system that looks like it will have between 20k and 100k rows in one of its tables generated per user, per year. How many users could one MySQL database be reasonably expected to handle in this kind of situation? Is MySQL the proper solution?

depends on the server (its type, configuration) and network infrastructure, but anywhere between 1 and many thousands.

is it the proper solution, who knows. theres lots of rerasosn to choose MySQL, and equally not to shoose MySQL. it can be the right tool in the right hands, iot cna be a disaster, but so can virtaully all server DB's otuy there.

it depends what you want to do, what you knowledge of the db is, how good your design is and so on.

MySQL is perfectly capable scaleable DB, it has issues, personally I don't think the security model is especailly good or intuitive, but its realtively cheap. I thinkl the biggest bugbear, given the size of data you expect to store is going to be finding good, weel qualified competant DBA'a to design and develop the db.

As has been said the hardware, CPU, memory, disk layout all play an important role in optimizing the performance. Data access will also help in determining the accessibility of information i.e. read-only or read/write. There are so many factors that determining whether a database can handle the load.