The only thing you can be certain of is that you can currently install and activate your software on an existing system. The future is uncertain, and in this case it is definitely not infinite in duration. If the disk or system where that installation is running on goes belly up, and the online installer and activation system is no longer running, you will not even be able reinstall on your current machine. So it may be wise to get an offline activation file for your current machine and store that in a safe place as well. 
 
As far as the status of the intellectual property goes, it is only abandonware if the owner decides to make it so. There are reasons why a product that is not producing revenue might be kept locked up and jealously guarded, instead of being left by the curb with a "take me for free" sign. One is that people who have a claim on your property might think it has some value to them or someone else. If your house is being foreclosed, you are not free to burn it down or invite in squatters. It is possible that someone might buy SONAR for cheap just to package it with hardware, for example. Another is that the copyright protects, not just the owner, but his competitors as well. If a full featured monster DAW like SONAR were suddenly to be released free to all comers, it might have a serious impact on the market, at least until technology has moved far enough to make it truly obsolete. Had you been offered SONAR, in its current state of development for free vs Cubase for $600+, it might have cost Steinberg a customer this year. Many users (as many have said on this forum) could happily use SONAR for at least a few years to come. It might be worth it for a competitor to bid at the sheriff's auction with the sole intention of tying up the copyright so that no one could legally offer it for free.