I'm sure they won't, government has never made such a decision on an existing fleet, only on new procurements such as hybrid buses and now bi-mode intercity trains. We still have unmodified Class 47s and clapped out school buses and coaches plodding around, hundreds of Class 66s which are not compliant to current regs. Legislating to make an existing vehicle fleet compliant to current regulations would be unprecedented in this way, they did it with PRM but they are modest overfloor modifications with a huge public interest, not full powertrain replacement which has no visible impact for the consumer other than being without their train while it's being done!
Again, it comes down to what is made illegal. Government legislates ends, not means. If that end is to achieve a certain level of pollution in a city, then there are levers to achieve that and withdrawal of DMUs may be one, use of battery packs in hybrid DMUs close to city centres is another. The London high emission zone was legislated to achieve the legislated levels of pollution (unsuccessfully, so far), but the complete withdrawal of all high emission vehicles was not. Plus emissions have always been legislated on a European level and have never mandated the withdrawal or modification of a full fleet.
Sprinters aren't going to be made illegal and the cost of completely re-engineering existing Sprinters would be so high and with so little payback that it would never be done, they may as well just wait until they're replaced anyway. They could I suppose be restricted out of larger city centres in time but the cost would be astronomical and this process is happening anyway when they expire in a few years.
When new DMUs are needed, they'll have to be ordered to the current EU legislation, as per the 195s (and I don't expect any deviation from that after Brexit) and this achieves the same end that you're suggesting but in a much more pragmatic way.