Why can they not use inflatable finishing line? like they do at the flamme rouge? it seems like the structure is a scaffold structure? Also could they not use plastic crowd control barriers? or not have barriers at all and ban crowds from the final 250m of sprint finishes? then they could just use a tape or rope to designate the course?
Inflatable finish lines would be a much easier thing, definitely, but part of the issue I believe is that the structure needs to include various wiring etc. for the offiicial timing clock, the transponder recognition, and so on. Now, would it be possible to have that on an inflatable structure, or on a sturdier structure located within an inflatable barrier? Not sure. Could be a possibility.
I can't see banning crowds from the last 250m as being a realistic possibility, because of the need to marshal that. They'd need to have police on hand, an outright fan exclusion zone and so on, which would be very difficult to enforce and also absolutely kill the interest in the finale for flat stages as well as potentially create a crush when they open that zone to let people see the podium ceremony.
A couple of things they could do is create
wider finish line structures, so that they are set back from the actual barriers, so that if a rider is turfed off course at the line like Jakobsen was, they would be dispatched into a sort of apron/hard shoulder type area no different from a 'normal' crash, which could still be dangerous but is part of the accepted risk of the sport, and far less dangerous than being directed head first into a metal structure at 80km/h. A necessary consequence of this would be that it would necessitate finding wider, safer finishes. The flip side is that it would then limit options for finishes in non-sprint stages, where this would be a non-issue, such as mountaintop finishes and time trials. Therefore perhaps the structure could be telescopic, enabling it to be widened and narrowed as safety demands.
In smaller races barriers vary as issues. The País Vasco incident was actually necessitated because of a LACK of barriers; this was the same finish as used in the Vuelta stage won by Igor Antón in 2011, but because then the barriers extended for the whole final kilometre, those poles had been set back from the course and did not create any concern, so were not identified as an issue by either the organisers or the UCI until they cleared the traffic for the race and realised that with their shorter stretch of barriers, they were now a hazard that required a last-minute solution. Again, a potential solution would be to have either two sections of barriers - one conventional set, set back from the road to hold fans back, and a second, softer or banana-style as described before, to mark the edges of the course, with again an 'apron'.
Or, alternatively, there could be paint on the ground. This could take two forms: one to allow an 'acceptable race area' with the outside to be used as a run-off (the F1 solution), or to have a set of designated lanes that a rider has to pick going into the sprint lanes in, say, the last 200m, and then stick to them (the XC ski solution). But both have their flaws. The F1 solution necessitates wider roads, which isn't necessarily a bad thing for spriints, but may necessitate a categorisation of what stages are designated 'sprint' stages to need to provide these, and then you get into the debate of what size group is a sprint that necessitates such a demarcation, because we've seen some fairly sizable sprints on stages designated high mountain when design is poor, or atop un-selective cllimbs. And of course, since the phasing out of gravel traps and grass run-off in favour of tarmac, F1 has largely lost the simple accident, but seen a big spike in controversial stewards' decisions over what constitutes giving places back, gaining advantage, evasive manoeuvres etc. to justify being in the run-off area. And the XC solution has the problem that a) it's harder to pick your lane at the speeds sprinters will be going at, b) it might just be moving the danger 200-250m back down the road as people fight over the choice of lane, or veer wildly to find an open lane if they're being forced into a lane another rider already has so they can't pass them.
Probably in all honesty, the best thing that can be done is to provide better barriering, and better enforcement of rules and punishment of offenders, both organisers, teams and riders. Have perhaps a penalty points system for this kind of infraction so that riders who have been deviating from their line can be benched even if they haven't caused any accidents because of situations like the Bouhanni/Matthews one, and punish the team with fines for failing to control their riders. And for race organisers, if teams raise concerns about the safety of designs that are felt to be legitimate,
they get penalty points too, punishable by withdrawal of their races' status. Finishes like this are about the money, so hit them in the pockets to incentivise safer race arrangements.