Mirrorless Cameras: Are they good investment?

The following questions I ask because as they say, "knowledge is power". And the idea of a DSLR type camera only smaller sounds ideal to me.

I have seen so far 3 mirrorless cameras that I like. Canon EOS-M, Sony NEX-7, and Olympus OM-D EM-5.

1) Are the lenses for these or all mirrorless cameras able to use on the any updated version? Say I invest in the NEX-7 and get a good lens, will I be able to use the lens if they make a NEX-8? Well, the only way to know this is if you can use the lenses of the previous model that came before NEX-7 on the NEX-7.

I know more about DSLRs then I know about mirrorless cameras. I know I can invest in lenses on and Canon and Nikon. But what about mirrorless cameras like the 3 I mentioned?

2) What about a life of a mirrorless camera? I know that with DSLRs they have an idea of what is expected with those "shutter cycles test". But what about mirrorless cameras?

Thank you once again.

Added (1). Also, if you know anything about any of the 3 cameras I mentioned, please let me know what you think about them. And any information you know.

Digital cameras are never a good investment since you will be replacing it every three to five years. Camera systems are a good investment, since the lenses you buy will last for decades.

Unless you do not shoot action and sports, mirrorless cameras are fine.

Otherwise, if you do shoot action and sports, you need to look at cameras with optical viewfinders like the Canon 700D or Nikon D3200