Train hopping has always been a dangerous sport.
Camera system hopping is not so much dangerous as an extreme waste of money when done with out good reason.
The only logical driver for buying new or different kit, is to achieve something that your existing kit can not accomplish.
Even then it should be something you need to achieve, not simply to have the ability to achieve some thing that you will never need to do.
Few people have skills and needs that exceed the ability of a DSLR.
At the highest level of camera, In many areas, a Mirrorless exceeds the abilities of a DSLR, Only in a few they are still a little way behind.
It is perfectly reasonable to continue using existing DSLR sysems, However it is not reasonable to continue to buy into them, or expand them. As there will come a time in the next few years when support for them becomes more than problematic, and parts unobtainable. Digital cameras do not have the extended life associated with previous generation of manual film cameras. It is already impossible to find parts for many five year old digital cameras. Sensors for previous generations of digital cameras are no longer in production nor are the processors.
The choice between full frame and a Crop sensor is a different issue, but again comes down to needs. In many respects Both can provide as high a quality than a majority of Professional and advanced amateurs will ever need. A crop sensor now far exceed the quality that can be achieved on 35mm film. A FF sensor is more akin to the results achievable on Medium format or 5x4. And FF shares many of the disadvantages of those larger formats, namely, size weight, As well as larger heaver lenses.
Size and weight has become a well recognised issue, and is now a deciding factor in many photographers choices.
The marketing direction of most manufacturers has been toward Full Frame Mirrorless. However many of the popular advances have been incorporated in APS and 4/3rds mirrorless cameras. with the FUJI X T3 being the highest featured of them. while lenses for such crop cameras are far from inexpensive, they generally cost far less than their FF counterparts of equal quality.
In the real world, images produced by the leading makers of both crop and FF mirrorless cameras are indistinguishable from one another and from their DSLR counterparts.
However it is perhaps a truism to say that Mirrorless have the clear advantage in advanced abilities and convenience. and hold all the cards for future research and development.