I'm speaking from experience here. I've been through this before with other software. So right now there is mass hysteria. People are flying about all over the place panicking and buying dumb deals. So Cakewalk hasn't actually passed away yet. It's not gone cold, there is still blood pumping and it has a breath. Sonar is still likely to be perfectly usable for at least another year! All the rats are jumping from the ship before it has even sunk and before the life rafts have even been launched. Just calm down!
So I've been through this before more than once and last time I made a really bad decision because of it, so I urge you to just hold back for a bit and don't panic. I made a bad decision because a software I used, a development framework, suddenly found its self in exactly the same position Cakewalk is in now. It was all doom and gloom and was basically over. I joined the horde and migrated to another system as soon as possible. That meant a massive learning curve and a huge outlay of time spent redeveloping and porting to the new system and of course more expense. However, at the 11th hour, the software was saved and became even better for it! Rather than all the learning, time and expense, it cost me, I could have just kept my cool and carried on! OK, I didn't know it was going to be alright, but I should have done because I had been through the same thing before with another software company. Alright, it was open source software on that occasion but that makes no difference as there is always a large company behind and sponsoring open source software. There are still huge commercial interests.
So before you jump ship consider the following. You purchased Sonar. It still works. It will most probably work for at least a year which is essentially what you purchased. You have plenty of time to see what might develop without immediately rushing out and spending another load of money. What might develop? Well, it's still possible someone might purchase Cakewalk from Gibson. While Gibson might have privately offered it for sale to a few companies before the news broke, other companies have only just heard about this at the same time as you. Cakewalk and Sonar are big brands that have great software, a great team, a load of great customers and assets. Any slump in sales is down to management - Gibson of late. Gibson has huge financial problems and is about to die. If they can recoup anything by selling Cakewalk and its assets, they would do so rather than just bin it to cut losses. So Cakewalk isn't dead yet, and I would be surprised to see it die now. Wait and see. Then there is always the chance a phoenix could rise out of the ashes - some of the best software has come about from top brand developers being displaced and starting something new.
On the other side of things, if Cakewalk was to die in this tragedy, there are plenty of rival software companies ready to pick up the custom. They will happily offer ridiculous offers to get the dispossessed Cakewalk users in their customer base. It's worth a lot to them. The offers will come and they will be hard to pass up because they know that once you have been through the learning curve, you will stick with their product, make the brand more popular and buy into the updates and add-ons. The problem is right now Sonar users migrating to Prosonus are inflating the crossgrade prices. A company will sell its product for the highest price it can - it's supply and demand. They won't offer no-brainer deals if people are already snapping it up at the going rate. So again, chill out, see what happens, if Cakewalk lives we still have our Sonar. If it dies we still have our Sonar for a while and the crossgrade deals will become ridiculous. Jump ship now and you are just pushing the conclusions and driving the prices up.