texture like cheese / destroyed after decimate

Hello everyone 

For my cc i do 3D object with a scan on my phone, then from sketchfab i use them as obj on blender and from blender to sims4studio .... i'm a converter not a creator.

for a long time i used to convert other's work on sketchfab on heavy object and everything was perfectly fine... with mine it get destroyed by decimate them. (picture below)

So for exemple : this is an object i did with someone else work : it's almost 600K of triangles : sketchfab.com/3d-models/porte-montre-en-faience-d1b83bfbb68e4186b67eb2569c89611a
and here is my scan (my work) : for 140k of triangles (so less than the other one) : sketchfab.com/3d-models/djkeekekek-cd8ff0e0b04649f2a82512907f71c495

when i did decimate on the 600k to 20,000 triangles on blender, the texture was still flat and perfect.... when i did decimate on my 140K to 20,000 triangles... the texture was weird, triangles, hole, crispy.... why ? 
Do you have any recmmendation ? it s pretty close to ugly sometime but in blender or s4studio its pretty... in game destroyed.... i never do more than 20.000 triangles it s always fine in game juste the texture is strange.... can someone help ? some exemple below ...

64.media.tumblr.com/9b4de028c18ee3bbbb97f425adcff648/c665ca2e0254e56e-a1/s640x960/ed667fc3821ad0b2d3a92840fae14cdcdadba86f.pnj
64.media.tumblr.com/945691a1f277b2732f9b498772b37d72/c665ca2e0254e56e-fb/s2048x3072/3436c2db4534891339d044a3ebb81c41e6c481dd.pnj
64.media.tumblr.com/cab41a098c0c99cd6ab970fa8dabb323/c665ca2e0254e56e-91/s2048x3072/3aa8da538b5ce89ca6b09c42296b6fc2abc1409d.pnj
64.media.tumblr.com/c7dfd2f5160ddd33e5d234423b4d1f1d/c665ca2e0254e56e-fa/s2048x3072/bb7b0717f921ea5ea26d79cd499eeec6d5f36644.pnj



thank you a lot 
Rudy