The UK is quite unique as it was the second largest contributor and also had the 5th largest economy now slipped to 6th in the world, in reflection the UK leaving is equivalent to that of 18 of the poorer EU nations in economical value to the EU, it is highly unlikely that this would create a domino effect simply because most of the EU nations will not be in any foreseeable position to do so, only France and Germany could theoretically make a success or those not tied so tightly into the EU, if either France or Germany did leave then the EU seizes to exist in it's current form.
Many people fail to recognise the very dire financial position the EU are in, a trillion Euro bonds buy up which prevented the Euro collapsing with the financial crash did not stimulate the EU economy like it was supposed to and now that tap has been turned off we see the EU going through economical slow down we went through at the beginning, the only difference is we have benefitted the UKs financial situation and are now able to inject it back into the system, the EU however has crippled itself and has no way of recouping the bonds buy out, Corona virus on top of this will only see more bonds bought up and we are not far off the Euro collapsing, this will destroy many EU economies and we also will feel the knock on effect but been external to the Euro and EU will act as a cushion effect....
Unless the EU drastically changes its destination and goes into a state of long term austerity to counter the inevitable, it is blindly walking into a disaster that will ripple around the world.