In my view, the COM/NET/ORG/INFO versions of the same generic, non-trademarked keyword domain running websites in the same industry, are just like having four MacDonalds restos on the same street next to each other, owned by different franchisees.
What that scenario does, is simply pull away customers from each other... thereby reducing traffic from every store, and thus reducing potential income. For an "end-user", such prospect dilutes his business position.
This means, if a legit business grabbed the .NET version and built his brand into it, what it does is reduce the "end-user" commercial value of the other domain extensions next to it that are not yet developed.
In my view, why would i buy Bananas.COM, if there is already a legit site Bananas.NET, and we are both selling bananas ???? It would dilute my business position, and shatter my branding goals. And besides, the argument that websurfers are more inclined to trust the .COM domains compared to .NET domains, is an old-school thinking that is now getting eroded in the minds of new generation of websurfers.
And it is not even guaranteed that if you buy this keyword domain, you will automatically rank on top of Search results.
In my Google, if i search for "PERFUME", what ranks on top is "theperfumeshop dot com" ... a less than perfect domain, having been dropped twice based on Whois records. But it's on top of search anyway. That's what matters. The guy bought the domain for reg fee and built a strong e-commerce website-- without paying the domainer who owns the root keyword version.
Another example is PIONEER.COM. All the extensions are taken. And they use the domains for various unrelated business. And yet, if i type "pioneer" on Google, what shows on top on my end is "Pioneerelectronics.COM". Had they chose to get involved with all the Pioneer wannabes by getting any of the Pioneer keyword exensions, their "domain branding" would have been diluted.
So in summary, in my view, if a legit business grabbed the .NET extension and decided to build their brand on that extension, it will reduce the value of the .COM domain you are peddling by turning away other potential "end-users" who plan to go on the same business as the guy who owned the .NET domain.
Your remaining hope is an "end-user" who would use your keyword domain for a totally unrelated business than the .NET neighbor. Or perhaps a "disruptive" end-user who will buy your domain simply to confuse consumers and hope to profit from wayward traffic coming from his next-door .NET legit neighbor.