Next you will demand the Italy, Spain, Greece follow the rules of the EuroZone. /s
From what I understand:
An Eurozone forces, necessates, all members to have a similar productivity increase. Being within the Eurozone helps converging (and also growing faster) as long as the difference in kind of economies and financial cultures is not too big. If that similarity is good enough, losing the freedom to handle a big crisis with your own Central Bank and currency is less important than the steady advantages.
However... I think that for the long term healthy growth Spain, Portugal and Italy should all get out of the Eurozone before there is some new global crisis. Greece not yet.
And the sad thing here is that when these countries started during the 90ies to converge their currency to a fixed rate with the DeutschMark, giving up that constant devaluation, it could already be seen from economical indicators, that it would not be good for growth.
Optimism in makeability was imo too high all over Europe, and the path chosen was just followed.
(and considering the history of periods with criminal high inflation peaks, very much detrimental for business and upsetting ordinary people, I can even understand it)
Getting out of the Eurozone will allow them to pick up their old strategy of slowly devaluating their own currency at the same rate as their productivity (on labor and capital) that does not grow as fast as the northern Eurozone countries.
From what I saw in a financial investors risk article on expected currency rates of the New-Peseta, the New-Lira, the New-Escudo, it would go rather smoothly for Spain and Italy (no disruptive devaluation at the start and hardly weakening thereafter), but for the New-Escudo it was estimated that the New-Escudo would devaluate within 10 years to 50% of the value at the start of the transition and then slowly devaluate over time. Already quite disruptive.
Greece not yet, because in that same risk analysis the New-Drachma would devaluate within 10 years to roughly 25%. That is too much disruptive for now.
EDIT ... risk management of Big Investors does include these break-out scenarios as standard assessment: you do not want to have invested too much in countries where after a break-out the currency devaluates, nor do you want money parked on banks there.
The other thing is that the more a country has breaking out of the Eurozone stated by leading politicians, the higher that risk is assessed, the lower the appetite to invest in that country.
However... that the UK kept its own Pound... I am not that sure it was economically the best way forward for the UK. The kind of economy and financial culture fitted well enough with the North Eurozone countries.
But considering how important that feeling of independancy is for many in the UK, all over the society... I can understand it.
It also depends on the amount of trade. The more trade as % of GDP you have, the more disruptive up-down currency changes are !
The more your domestic economy, your domestic companies (!), are needing imports to be able to function, are needing imports to be able to export, the more disruptive up-down currency changes are.
And this B2B trading in components has increased enormously over the decades because of our more complex (specialised) economies.
Trade is no longer some mineral resources for the industry + some national specialties for consumers from complete watches, cars, to flowers or vegetables ! Around 60% of the UK imports are B2B needed for UK industry to function, including producing export goods. Around 30% of the value of goods exported by the UK is from imported goods (mostly from the Eurozone).
For my small country leaving the Eurozone would be a brain-dead stupid decision.
https://www.bbc.com/news/business-47212992
A very big country (with lots of domestic trade between regions) has less trade as % of GDP with other countries, and is less disrupted. If you add inter-state trade within the US to foreign trade, the trade as % of US GDP is comparable to the EU.
A small country the size of a region of a big country has two layers of trade: inter-regional trade + truly international trade. The EU, the Eurozone is mainly (in volumes) faciliating the (inter-regional) trade with direct neighbors.
The UK is not as big as China or the US and is fully dependant on that 60% imports to keep its own economy functioning. And the Pound to Euro volatility disruptive and also hindering the economical synergies from further "regional" trade, because low profit margin goods are too vulnerable for currency changes. Also one of the reasons that the trade between the UK and the EU is not as high as it could have been.
One of the projects the EU has running is the potential of further eliminating borders to tap into the economical potential of "trade" between small companies and public services (like hospitals) across borders.
Similar to the intertwining across the Irish border we have now and at risk by Brexit. That EU growth potential with not too big efforts, though a long term program, was estimated at Euro 200 Billion additional GDP.
One of the most fundamental reasons of the EU to come with the Eurozone was tho encourage companies to trade more within the Eurozone,
making the Eurozone (the EU) less dependant on international trade.
At the same time making the Eurozone more acttractive for international trade partners because of price stability between Eurozone countries from 1 currency, instead of always differing prices over time from a multitude of changing currency values.
Making the EU more attractive for international trade partners... meaning making it easier to get better FTA's with bigger quotas at reduced tariffs for exports of the EU.
The UK missed all that.