That's because they're not mutually exclusive arguments. And I still think calling it incredible luck to find a #1 WR in later rounds while not saying the same about great OGs is a ridiculous proposition. I can name as many current #1 WRs drafted outside the first round as a person can name current great OGs.
Regardless, the argument for the offensive tackle is:
1. He satisfies a current major need by playing OG
2. He satisfies a future major need by playing LT
3. He's a top tier talent at both positions
So I don't quite understand how you think both arguments aren't valid. As I said, they're not mutually exclusive events.
I didn't say the two arguments were mutually exclusive, but it's a way to keep the goalpost moving around while peppering in silly things like "Watkins won't protect Sam!" as if the plan was to totally ignore the OL if we didn't take a tackle first. If only THAT nonsense can be canned, I think the air can be cleared a little around here. EVERYONE wants to address the OL. We just have different ideas of how to do it, and we have different ideas as to the rarity of really good guards vs. #1 WRs.
But I will add a reason to your list:
4. The Rams either feel Watkins is NOT a special #1 level guy, OR they feel that Evans IS and plan to draft him in the neighborhood of #13 OR they sign someone like Hakeem Nicks.
Under any of those scenarios, the only two sensible solutions left IMO are draft an OT to play OG (hopefully not long term, and hopefully Matthews IMO in that scenario) or draft out of the top ten completely and stack up multiple hopefully very good players instead of one hopefully elite player.
And of course there's also the idea the Rams pull some razamatazz (that IS the technical term for it) and draft BOTH... which would make go "Huh."