If you are intent on using your old software then it might be wise to keep that old computer as well just to run it.
Personally, I prefer to VM an older OS. Why keep older hardware around when the real value is the software itself? Keeping that older hardware running can be more cost prohibitive then it's worth.
I understand the consequences of accumulating monthly payments but you have to consider what the payment is for!
This is for your lively hood and is a crucial tool for your work. You can easily cover that if you make any money off of it IMO.
If there is an alternative and it doesn't sacrifice quality and efficiency for the end product (in some instances superior, in some lacking) and it keeps ones cost down, wouldn't it be in the best interest of the business to explore that possibility?
Just because a decision was good 1, 5, 10, 15, 20 yrs ago, doesn't in of itself mean that it's still a good decision.
I have ended up ditching proprietary sign software and am using Adobe software now.
To me having updated software has more advantages than relying on past versions. (i receive artwork from outside sources quite often)
I have no more hassles (or very few) when opening files.
One proprietary system for another. Now, I'm not saying that the software isn't good or is bad. It may very well be the best option for one to use, but it would behoove one to actually investigate every so often to make sure that's the case.
Oh and just because it's the latest and greatest doesn't mean that it's always the best. Functions get deprecated and or removed. Functions that the business may need to operate. So just because it's the latest doesn't in of itself mean that it's the greatest.
Being "bleeding edge" (which people are with Adobe's new update schedule of no older then x-1) isn't always what it's cracked up to be.
Now, one thing that I would say that already puts at least me in a different camp then you (and quite possibly be the case for others) the "outside" files that I get, they aren't Ai files. They aren't DRAW files. They may be a PDF file (with a raster file embedded in it). So that's at least one thing that makes my needs different then your own. Just because your needs may or may not allow you to switch, doesn't mean that it's the same for the next person.
There's nothing wrong in keeping your old software if it is still doing what you need it for but my opinion is don't ***** when you can't open files that are given to you.
If that happens then take a deposit on the job and get a subscription ASAP!
Ditto when being "bleeding edge" bites one as well (I see far more people complaining about the most recent whatever update in order to run bleeding edge, then with keeping slightly older software (still supported, I'm not talking legacy software here).
Adobe CC costs over 80.00$ canadian a month right now (varies with the exchange rate) and i couldn't be happier using it.
Bare in mind some of their subscription packages, they doubled the cost for some customers (not all, only some IPs saw it when they visited the site). Increase, increase, increase and there is no promise that a feature that one enjoys today will be there tomorrow. The joys of being "bleeding edge" on a rolling release schema.