The plan is indeed in the long term for Simutrans-Experimental to have a stable version (but it will have its own stable version, rather than replace Simutrans-Standard - Simutrans-Experimental is a fork). However, before we can get to that point, we need to finalise the develoment of a Simutrans-Experimental compatible pakset so that (1) all the features that require a fully compatible pakset can be tested; and (2) the game balanced precisely (both in the code and in the pakset) before a stable version is released. Until then, no major new features are being added to the code that are not merged in from Simutrans-Standard (which is done regularly as Knightly observed), and revisions of the code are limited to: (1) fixing bugs; (2) optimising; (3) minor enhancements to the UI; and (4) improving the game balance (hence the recent re-scaling feature).
When a stable version is released, the aim is to have a complete automated installer package with the binaries, information on how to get the source code (it is on Github), and at least one fully compatible pakset all included. This would be an .exe installer for Windows and a .deb / .rpm installer for Linux. Hopefully, there will be a Macintosh .app installer, too.
As to the name, "Experimental" is not fitting for a stable version, so, while I understand what Hajo says about confusion, I think that greater confusion would obtain in the long-term if stable software were named "Experimental". I really do not want users put off by thinking that the software is not a stable version. The plan is for "Simutrans-Extended" to be the name of the stable version, and for any major new features to go into a sub-branch "Simutrans-Experimental", which, when that is stable, will then be merged into Simutrans-Extended, which will then be the stable branch.
That process is some way in the future yet, however, as work still needs to be done on producing the fully compatible pakset, which might take some time (I am adapting the open-source Pak128.Britain to make a Simutrans-Experimental branch of it: see
here for details - volunteers to assist would be greatly appreciated, as the process would be hastened).