Naturally, the motivation for an author to further continue on an item depends directly on the success (or lack thereof) for that item, and the only measurement for that is total sales number and/or sales revenue. But at the same time, every sale is a business transaction where both parties get exactly what they bargained for (buyer gets the item, seller gets the money) … no one side is doing the other one a favor here, that could be “called in later”; so to turn around and state that the buyer is somewhat entitled to get any more than what s/he purchased, even if the item is discontinued, simply has no foundation.
Is it fair for some authors to quickly remove items for sale and simply re-release another item that looks, feels and works almost the same? Of course not, and as far as I know, market regulations actually stipulate that such a behavior is not allowed. So if a seller of yours is discontinueing and re-packaging an item and sell it as “new”, you should contact Envato and have them look into it. Although, at the same time, some superficial similarities between items does not automatically mean the items have much to do with each other.
And it is not uncommon that author and some buyers work together to make a product better (not talking about bug-fixing but adding new features). But again, this can not be construed as if the buyer is doing the author a favor, where the author “owes” the buyer anything in return. If the author manages to incorporate any changes or ideas a buyer proposed, the buyer got exactly what s/he wanted, without having to buy another item, or the same item again (due to lifetime updates). And the author has an improved item that can possibly attract more buyers, therefore increasing overall sales. Both sides win here, and in my opinion, this is a very fair trade-of. If an author takes such a proposed idea, but does NOT include it into an existing item, but instead decides to release a new one, the story is somewhat different of course, at least ethically speaking, because there is no legally binding agreement between author and the buyer proposing/co-developing a new feature, that stipulates that the new feature must be incorporated into an existing item.
The prices here on Envato are exceptionally low (compared to many other market places), so the only way for an author to make a living out of selling here, is by selling a lot. While that is particularly true for plugin and script authors, it also applies to some lesser degree to theme authors (although themes usually run 4 - 5 times the price of a decent plugin). But at the same time, most successful theme authors are actually teams of multiple people, where revenue has to be share, while plugin/script authors are usually single freelancers.