In a normal year, I'd agree with you. We'd look to add a veteran RT, so that going in the draft, we're not pigeon-holed there. This year, however, there's so little cap space available, and a good chunk of it may be going either to retain players we already had last year (pass rushers) or to extend Lamar. In which case, the ability to add a Tackle in FA becomes that much more challenging. In addition, I've looked at the Tackle FA market, and as it stands today, I don't know that I remember a year worse than this one to need a Tackle in FA. That could change with roster cuts, but that's what I see today. Its total garbage. Like Fluker looks good in this market.
The added advantage of keeping Brown another year, knowing you have the same problem next year, is well, another year. You can two drafts to add Tackles instead of one, and you get two FA periods. More money likely to be available next year, and it would be hard for it to be a worse FA class than it is now.
If Orlando Brown were a FA this offseason, given how bad that market is, he would get AT LEAST $20M annually, and I'm not convinced he wouldn't be the highest paid Tackle in league history. And I don't think he is the best Tackle in the league, but the market is just so egregious right now.
So regardless of whether they trade him or not, I think they need to add two Tackles this offseason into the mix, through a combination of draft picks and/or free agent signings.