If you would find a eARC splitter and use it as you’ve pointed out, the Sonos Amp would send out the same sound as the Arc Ultra. What would be the benefit of this?

I dont want to live without towers in my home cinema, and i love music so much as movies. Towers speakers are just another level and i love the easy life of sonos. For me is just the best level of home cinema

Unfortunately, running towers along with a soundbar is going to mess up your front soundstage and make any directional cues muddied and spread out, due to the repeating of channels that are supposed to be discrete.  If I were you, I’d keep the towers for music and leave the Arc and surrounds alone.  More speakers does not mean better sound. 

But imagine if i get towers. era 300 and a sonos sub and i program to have in home theater mood. Sonos arc ultra and a sub and programed like a second home theater mood. With a earc splitter it can run without any problem...? Im not looking for power here i just like to have front left and right spekears 

You already have left and right front speakers in the Arc; Left/Center/Right speakers in a single enclosure is the very definition of a soundbar.  Adding additional left and right front speakers is going to mess up your front soundstage and muddy the directional output. 

Once again (and I cannot stress this enough) - More speakers does not equal better sound!

Then if sonos dont release towers speakers i will get a traditional home theater 

Well that’s a whole different request.  Not one I see Sonos doing (it’s been requested since the Playbar was released over a decade ago), but it is far different from splitting an HDMI-ARC signal to both a soundbar and two towers. 

Soundbars, SONOS or not, are intended to install quickly without a lot of “boxes”, wires, and complicated options.