I do think emulators are MSX preservation in the sense that they capture the details of various machines, components and extensions, so that this knowledge is available to everyone and is not lost as MSX computers (esp. rare ones) become harder and more expensive to obtain. It’s a form of documentation. But I guess this is about semantics and your and my interpretation of the word preservation is likely different.
I understand your point. But this is not keeping the MSX alive, only saving (documenting) the memories of something once called MSX. It's something like those seeds that are being stored to preserve some plants before they become extinct. To truly keep the MSX alive we have to keep using it before it vanishes, not only "storing the seeds" and being comfortable with that. Iup! Philosopher's day! ;-)
Finally, sure, MSX are now harder to find than 30 years ago but... we are less too. Everyone that wants an MSX nowadays can find it easily. They are more expensive than in the 80s? Well... that's right, back in the 80's my parents bought it to me, so it was sort of free from my point of view 
Emulators are indeed not indispensible to develop, I could program fine when my MSX was my only computer in high school after all. However, for me at the moment they are indispensible in the sense that I would not develop for MSX without it (for earlier mentioned reasons).
(p.s. When I was developing software on the MSX in high school and early university, most of my projects were not completed whereas now they are, despite that I have much less time to spend on MSX nowadays. Hmm, I wonder why.)
As you told before (I think it was you), now you are a professional developer. So, there is another difference between your childhood and your present age: now you have tons of knowledge and experience you didn't have before. So, another reason (probably the most important one) for you to finish more projects now than before is... experience and knowledge. Not just emulators. And I am sure that most of current MSX developers are in a situation like yours.
We are also mixing "cross development" with "emulation"
The development machines had a Z80 board (could be swapped out for other CPUs) to test and debug the Z80 code on, without deploying to the MSX, so they were more than cross development alone. Maybe it does not fall under the definition of emulation, but it’s similar in concept, to run code meant for MSX in an alien environment. The Konami developers would receive a similar amount of "the spirit of msx" as I do when I develop my software with openMSX.
Yes. Emulation is useful (though not indispensable) in that sense. Emulators are useful (though not indispensable) for development. But in my opinion, they are not useful to preserve the MSX. Let's imagine an alternate universe in which, back in the 80's, those powerful cross-development machines had been freely available to everybody... would the MSX exist nowadays?
