Trump already has the support of his base, who are the people most likely to be strongly supportive of his position here. The question becomes, does making this statement persuade any ADDITIONAL voters to support Trump? Or does it push some who are reluctantly supporting Trump away? IMHO, while this might energize MAGA nation, it doesn’t help him expand his base beyond that, and may instead push those on the fence away. Remember, most of those people who are persuadable to vote for Trump but are not strong supporters are more likely to be influenced by the media. This is not some issue where it can be reasonably portrayed as the media doing a hatchet job on Trump, lying about him or persecuting him - these are his words, spoken in a very public venue.
It all comes down to a risk/benefit analysis. We can all see the risk of Trump making this statement, he will be portrayed as an isolationist, as being pro-dictator (yes, Putin is a dictator), being a loose cannon in foreign affairs, and risks alienating independent voters who might be supportive of NATO. So what is the benefit to his campaign, other than getting the base worked up? How does it increase his chances of getting elected?
It does lower the ceiling of potential maximum support, yes. Especially given the press will exploit it - they were handed ammunition they didn’t even have to fabricate.