Baker is excellent, a very under-used presenter. He is clear, authoritative, informed, and is not an overbearing screen-hogger. I like Packham a lot on Springwatch, but Baker could be better. Although a lack of antics and japery might be seen as a bit dull. Here's an idea, replace Martin with Nick Baker? Martin's ok, but he doesn't seem to know very much.
I will miss Humble, as she was ideal as the 'everyman/woman' asking the questions on behalf of the public. Strachan, sadly, is very poor and seems out of place to me. Alice Roberts or Charlotte Uhlenbroek could be good, but I expect they are overqualified and would not be keen.
I do wonder if Humble leaving will kill the dynamic, and spell the end for Spring/Autumnwatch? Viewng figures seem quite high still, but I for one find it harder each year to get enthused about the same badgers, red deer, foxes, nestbox birds etc. They do seem to have been almost everywhere now, and look at almost everything.
I agree except Martin also has a degree in Zoology so he's not as dense as he likes us to think he is.