SaaS is the way everyone seems to be going these days, as it is a huge revenue producer for software vendors. Why make $50 once when you can instead milk someone for $10/month for life? As a holder of multiple desktop licenses for products like Quickbooks, Photoshop, etc I can say that I HATE when I am forced to go to a subscription-based model unless it comes with some very good perks. Quickbooks is using pretty brutal tactics to get everyone to switch, including disabling real-time sync with bank accounts, etc. For this reason I will probably switch to a different software vendor in the very near future.

If you do decide to go to an SaaS model, make sure that you give the users a very good reason to switch and ideally give them a choice, don't force it on them. Free upgrades are petty much a given, or else what are they paying for?