Veeam Availability Suite is not a "range of products", but rather a combination of the two specific products - Veeam Backup & Replication and Veeam ONE - at a heavily discounted price comparing to purchasing them standalone.
I am not quite sure what do you mean by "year on year 15-20% increases". If you are talking about yearly maintenance, then it is calculated as a percent off MSRP, for which we did not have 15-20% increases year on year. From what I remember, the increases so far have been around 10% on average in USD, and not every year even. These increases are required to keep the effective pricing the same as new CPUs with more and more cores per socket are introduced into the market, resulting in less and less sockets needed to run the same amount of VMs (while as you know, Veeam licenses per-socket).
Anyway, as far as the justification - this maintenance fee covers our in-house technical support costs, and entitles you to upgrade to new versions of the products that you already own, thus providing you access to all the new functionality at no additional cost. Judging on our Renewal Team never missing their target, the majority of our customers are happy with the value they are getting for the money.
Paying maintenance for Veeam products you already own does not automatically entitle you to get new
Veeam products at no cost. However, as the existing customer you may be eligible for some promotion - for example, the one that has just started
provides production licenses for Veeam Backup for Office 365 (for 1 or 2 years depending on B&R edition you own), as well as 6 months subscriptions to Veeam Agent for Windows and Veeam Agent for Linux.