Not necessarily. There are plenty of shows on TV, including on CBS, where the male and female leads are never treated as potential romantic partners. There was never any romantic subplot between CSI's Grissom (or Langston or Russell) and Willows, or between CSI: NY's Mac Taylor and either of his female leads, or between Goren and Eames on Law & Order: Criminal Intent, or between Reese and Carter on Person of Interest. We're well past the old days when it was assumed that female characters could only be love interests.
On the other hand, I did see a hint of a "chemistry beat" between them in one of the clips in the preview. And heck, it's not like the implication of romance between Holmes and Watson is anything new, at least where some fans are concerned (see Moffat's Sherlock and the running gag where everyone assumes Sherlock and John are a couple).
I think that's true of ensemble shows like the CSIs. However Elementary, I would imagine, will be more akin to The Mentalist, Castle or Bones, in that there is a distinct male and female lead, with a group of peripheral characters supporting. Under those circumstances I'd still wager that at some point, we're going to see it. Personally, as per above, I think they'd have got better narrative mileage out of casting a black Holmes than a female Watson.
Re the second point, well yes, there is that!
Like I've said, there are plenty of CBS shows that are not built around "ships" between their male and female leads. So I don't see any reason to conclude that here. Don't you think it could've simply been out of a desire to be more inclusive? After all, they cast an Asian actress as Watson and a Latino as the Lestrade equivalent. Maybe they just dislike the idea of a show without a central female character.
Well I think there's definitely truth in the last point I think. It's almost like they have a checklist - male lead, check. Female lead, check. Ethnic characters, check. Again, I'd rather have had a black Holmes, a male Watson, and take the 'buddy' route, casting off any temptation to play the shipper angle which has been done to death, and continues to be done to death.
Why does there have to be a limit to how many changes they make? Especially since there's a competing modern-Holmes show on in the UK at the same time, and the producers of that show have hinted they might raise a legal challenge if the shows are too similar. So the producers of Elementary have an incentive to differentiate it from Sherlock as much as they can.
Despite the sabre-rattling there's only so much that the BBC and Moffet could actually do as almost all of the original narrative is now in the public domain in the US. Any similarities that exist between the two shows that are derived from the source material would be fair game. If, however, their version took Sherlock, turned him black, made Watson female, had them as lovers, transported the story to outer Mongolia and had them consulting for the Mongolese secret police, and so did the US version, then they may have a case.
As it is, the only similarity between the two shows outside of the source material is that they're both set in a modern context. It's unlikely that the BBC would get anywhere with any legal challenge with that as their sole foundation.
Does there have to be a limit on what changes are made? Of course not, but if I were a Holmes fan, or had interest enough to go out of my way to watch this then I'd expect to recognize certain fundamentals.