Decimating without destroying mesh.

Hi. Are you sure they are holes and not flipped normals?
Try this:
1). Switch to Edit mode, select everything with A, Tools - remove doubles with Merge distance set to 0,0001. This will merge vertices along seams to prevent holes from appearing there
2). Use Decimate, apply
3). If there are holes in random places, try this: select everything, Mesh - Normals - recalculate outside

There are tools like Instant meshes addon, but it will not preserve the details well

I managed to reduce from 90k to 20k but I got some ugly normals around the arms and I don't know how to fix them. I think it's because of the way decimate tool reduces triangles and make them messy. I tried to make a 40k version but it ended up the same. Is it possible to fix?

Image BEFORE/AFTER: www.dropbox.com/s/zdspe16nfytw53k/04-12-20_2-53-47%C2%A0AM.png

Another method to reducing the polycount of a mesh is deleting some of the edge loops by hand which gives you a little more control over the way the mesh is reduced

It depends a lot on the mesh, but if the topology isn't too wrecked to begin with, I use edge loops to reduce polys in a mesh. This works best where the topology is even and is easily editable, and there are few or no holes. I prefer to not use the Decimate tool because it ruins the topology and makes a lot of holes in the mesh if you're not careful.

I use Alt+J to make triangles into squares, and when there's just square polys left, I use edge loop selection on loops, click X, select "Dissolve edges" and the edge loop is removed.

The method takes more time, but gives a much neater result and is a lot easier to edit later. 


EDIT:
Just an example shown on a sphere:

If you only have triangular faces, mark the mesh and click Alt + J to remove the extra lines (it only removes the lines where it "sees" square polys, and it's not always an exact science, so be prepared to do a little bit of fixing here and there). Edge loop only works properly on rectangular polys. Keep in mind that the triangular polys will be added back in the finished mesh when imported back to S4S. 
i.postimg.cc/Pr5L3Fg9/Screenshot-New-161.png - first mark every other horizontal edge loop (hold alt and R-click to select the first loop, then hold alt + shift and R-click to mark more edges), click X, Dissolve edges. 
i.postimg.cc/WbYdQwj3/Screenshot-New-162.png - Select vertical edges, do the same. 
i.postimg.cc/2Sq1mL8x/Screenshot-New-165.png - Did it one more round
i.postimg.cc/sgtv2fMC/Screenshot-New-164.png - Smoothed the mesh, it's the same as 165.  
Polycounts can be seen at the red line. This took me less than 5 minutes, but it was a very simple mesh. Can take more time with more complicated meshes. 

I've used this method for quite a while, and it works as long as you can delete edge loops without deleting the faces. You have to pay some attention to edges in UVmaps (UVmaps can get borked if you delete edges with seams), and more complicated meshes have areas where edge loops won't connect to themselves so the trick won't work, but with a bit of trial and failure this trick works quite well wherever there is a closed edge loop, and the topology and UVmap seems to do very well with this method with a bit of care. 

I have used this method to make meshes upwards of 500.000 polys (or more) into less than 20.000 while keeping most of the details and not borking the mesh, so it's absolutely worth looking into if you want cleaner but still detailed meshes, and don't want to use Decimate. I only use Decimate if the topology is already ruined beyond fixing... 

The best method is of course to start low-poly and not add more polys than strictly needed, because then you odn't have to reduce the polycount, but that might be too much to hope for.