There is no such tool or plug-in from Adobe. If you are the system administrator and need or want to move these users to InDesign, you need to factor in the cost of obtaining the third party software to convert documents where you believe that is necessary.
Q2ID is worth every last dime.
So - is there now a good solution (like an update, plugin or
something like that from Adobe itself) to get our users to
switch to InDesign (without being a special converting tool)?
Dov and Bob agree on the best answer.
However, if your clients have their old license information, perhaps the clients can borrow a set of old QXP disks, and use their valid licenses.
Alternatively, it's likely you can find the disks for sale on the Internet. The usual caveat is that these purchases are often illegal copies that use illegally-broken license. If your clients have their old license information, even if the disks are illegal copies, it probably wouldn't be illegal for thm to use their valid licenses to install QXP from such disks. You might want to talk to an attorney about legality.
Another good resource is to buy stuff from dead or dying print shops. In addition to tons of equipment out there many print shops that are shutting their doors sell their computers and software. You can get some pretty smoking deals on old software, RIPs, platesetters and the like. Looking for nearby print shops shutting their doors is a great way to get some cheap equipment. They are shutting down all the time too.
Also great place to get some office furniture